Asymmetric conspecific sperm precedence in relation to spawning times in the Montastraea annularis species complex (Cnidaria: Scleractinia)

In broadcast spawners, prezygotic reproductive isolation depends on differences in the spatial and temporal patterns of gamete release and gametic incompatibility. Typically, gametic incompatibility is measured in no-choice crosses, but conspecific sperm precedence (CSP) can prevent hybridization in gametes that are compatible in the absence of sperm competition. Broadcast spawning corals in the Montastraea annularis species complex spawn annually on the same few evenings. Montastraea franksi spawns an average of 110 min before M. annularis, with a minimum gap of approximately 40 min. Gametes are compatible in no-choice heterospecific assays, but it is unknown whether eggs exhibit choice when in competition. Hybridization depends on either M. franksi eggs remaining unfertilized and in proximity to M. annularis when the latter species spawns or M. franksi sperm remaining in sufficient viable concentrations when M. annularis spawns. We found that the eggs of the early spawning M. franksi demonstrate strong CSP, whereas CSP appears to be lacking for M. annularis eggs. This study provides evidence of diverging gamete affinities between these recently separated species and suggests for the first time that selection may favour CSP in earlier spawning species when conspecific sperm is diluted and aged and is otherwise at a numeric and viability disadvantage with heterospecific sperm.

Contemporary evolution of sea urchin gamete-recognition proteins: experimental evidence of density-dependent gamete performance predicts shifts in allele frequencies over time

Species whose reproductive strategies evolved at one density regime might be poorly adapted to other regimes. Field and laboratory experiments on the sea urchin Strongylocentrotus franciscanus examined the influences of the two most common sperm-bindin alleles, which differ at two amino acid sites, on fertilization success. In the field experiment, the arginine/glycine (RG) genotype performed best at low densities and the glycine/arginine (GR) genotype at high densities. In the laboratory experiment, the RG genotype had a higher affinity with available eggs, whereas the GR genotype was less likely to induce polyspermy. These sea urchins can reach 200 years of age. The RG allele dominates in larger/old sea urchins, whereas smaller/younger sea urchins have near-equal RG and GR allele frequencies. A latitudinal cline in RG and GR genotypes is consistent with longer survival of sea urchins in the north and with predominance of RG genotypes in older individuals. The largest/oldest sea urchins were likely conceived at low densities, before sea-urchin predators, such as sea otters, were overharvested and sea-urchin densities exploded off the west coast of North America. Contemporary evolution of gamete-recognition proteins might allow species to adapt to shifts in abundances and reduces the risk of reproductive failure in altered populations.

The Tegula tango: a coevolutionary dance of interacting, positively selected sperm and egg proteins

Reproductive proteins commonly show signs of rapid divergence driven by positive selection. The mechanisms driving these changes have remained ambiguous in part because interacting male and female proteins have rarely been examined. We isolate an egg protein the vitelline envelope receptor for lysin (VERL) from Tegula, a genus of free-spawning marine snails. Like VERL from abalone, Tegula VERL is a major component of the VE surrounding the egg, includes a conserved zona pellucida (ZP) domain at its C-terminus, and possesses a unique, negatively charged domain of about 150 amino acids implicated in interactions with the positively charged lysin. Unlike for abalone VERL, where this unique VERL domain occurs in a tandem array of 22 repeats, Tegula VERL has just one such domain. Interspecific comparisons show that both lysin and the VERL domain diverge via positive selection, whereas the ZP domain evolves neutrally. Rates of nonsynonymous substitution are correlated between lysin and the VERL domain, consistent with sexual antagonism, although lineage-specific effects, perhaps owing to different ecologies, may alter the relative evolutionary rates of sperm- and egg-borne proteins.

Post-mating prezygotic barriers to gene exchange between hybridizing field crickets

Studies of sexual selection in speciation have traditionally focused on mate preference, with less attention given to traits that act between copulation and fertilization. However, recent work suggests that post-mating prezygotic barriers may play an important role in speciation. Here, we evaluate the role of such barriers in the field crickets, Gryllus firmus and Gryllus pennsylvanicus. Gryllus pennsylvanicus females mated with G. firmus males produce viable, fertile offspring, but when housed with both species produce offspring sired primarily by conspecifics. We evaluate patterns of sperm utilization in doubly mated G. pennsylvanicus females and find no evidence for conspecific sperm precedence. The reciprocal cross (G. firmus female × G. pennsylvanicus male) produces no progeny. Absence of progeny reflects a barrier to fertilization rather than reduced sperm transfer, storage or motility. We propose a classification scheme for mechanisms underlying post-mating prezygotic barriers similar to that used for premating barriers.

Phylogeography and bindin evolution in Arbacia, a sea urchin genus with an unusual distribution

Among shallow water sea urchin genera, Arbacia is the only genus that contains species found in both high and low latitudes. In order to determine the geographical origin of the genus and its history of speciation events, we constructed phylogenies based on cytochrome oxidase I and sperm bindin from all its species. Both the mitochondrial and the nuclear gene genealogies show that Arbacia originated in the temperate zone of the Southern Hemisphere and gave rise to three species in the eastern Pacific, which were then isolated from the Atlantic by the Isthmus of Panama. The mid-Atlantic barrier separated two additional species. The bindin data suggest that selection against hybridization is not important in the evolution of this molecule in this genus. Metz et al. in a previous publication found no evidence of selection on bindin of Arbacia and suggested that this might be due to allopatry between species, which obviated the need for species recognition. This suggestion formed the basis of the conclusion, widely spread in the literature, that the source of selection on sea urchin bindin (where it does occur) was reinforcement. However, the range of Arbacia spatuligera overlaps with that of two other species of Arbacia, and our data show that it is hybridizing with one of them. We found that even in the species that overlap geographically, there are no deviations from selective neutrality in the evolution of bindin.

Decoupling of rapid and adaptive evolution among seminal fluid proteins in heliconius butterflies with divergent mating systems

Reproductive proteins often diverge rapidly between species. This pattern is frequently attributed to postmating sexual selection. Heliconius butterflies offer a good opportunity to examine this hypothesis by contrasting patterns of reproductive protein evolution between clades with divergent mating systems. Pupal-mating Heliconius females typically mate only once, limiting opportunity for postmating sexual selection. In contrast, adult-mating females remate throughout life. Reproductive protein evolution is therefore predicted to be slower and show little evidence of positive selection in the pupal-mating clade. We examined this prediction by sequencing 18 seminal fluid protein genes from a dozen Heliconius species and a related outgroup. Two proteins exhibited dN/dS > 1, implicating positive selection in the rapid evolution of at least a few Heliconius seminal fluid proteins. However, contrary to predictions, the average evolutionary rate of seminal fluid proteins was greater among pupal-mating Heliconius. Based on these results, we suggest that positive selection and relaxed constraint can generate conflicting patterns of reproductive protein evolution between mating systems. As predicted, some loci may show elevated evolutionary rates in promiscuous taxa relative to monandrous taxa resulting from adaptations to postmating sexual selection. However, when monandry is derived (as in Heliconius), the opposite pattern may result from relaxed selective constraints.

Evolution of 2 reproductive proteins, ZP3 and PKDREJ, in cetaceans

The rapid evolution of proteins involved in reproduction has been documented in several animal taxa. This is thought to be the result of forces involved in sexual selection and is expected to be particularly strong in promiscuous mating systems. In this study, a range of cetacean species were used to analyze the patterns of evolution in 2 reproductive proteins involved in fertilization: the zona pellucida 3 (ZP3), present in the egg coat, and PKDREJ, localized in the sperm head. We targeted exons 6 and 7 of ZP3 and a part of the REJ domain in PKDREJ for a total of 958 bp in 18 species. We found very low levels of amino acid sequence divergence in both proteins, a very weak signal of positive selection in ZP3 and no signal in PKDREJ. These results were consistent with previous reports of a slow rate of molecular evolution in cetaceans but unexpected due to the existence of promiscuous mating systems in these species. The results raise questions about the evolution of reproductive isolation and species recognition in whales and dolphins.

Evolutionary patterns of two major reproduction candidate genes (Zp2 and Zp3) reveal no contribution to reproductive isolation between bovine species

BACKGROUND

It has been established that mammalian egg zona pellucida (ZP) glycoproteins are responsible for species-restricted binding of sperm to unfertilized eggs, inducing the sperm acrosome reaction, and preventing polyspermy. In mammals, ZP apparently represents a barrier to heterospecific fertilization and thus probably contributes to reproductive isolation between species. The evolutionary relationships between some members of the tribe Bovini are complex and highly debatable, particularly, those involving Bos and Bison species for which interspecific hybridization is extensively documented. Because reproductive isolation is known to be a major precursor of species divergence, testing evolutionary patterns of ZP glycoproteins may shed some light into the speciation process of these species. To this end, we have examined intraspecific and interspecific genetic variation of two ZP genes (Zp2 and Zp3) for seven representative species (111 individuals) from the Bovini tribe, including five species from Bos and Bison, and two species each from genera Bubalus and Syncerus.

RESULTS

A pattern of low levels of intraspecific polymorphism and interspecific divergence was detected for the two sequenced fragments each for Zp2 and Zp3. At intraspecific level, none of neutrality tests detected deviations from neutral equilibrium expectations for the two genes. Several haplotypes in both genes were shared by multiple species from Bos and Bison.

CONCLUSIONS

Here we argue that neither ancestral polymorphism nor introgressive hybridization alone can fully account for haplotype sharing among species from Bos and Bison, and that both scenarios have contributed to such a pattern of haplotype sharing observed here. Additionally, codon-based tests revealed strong evidence for purifying selection in the Zp3 coding haplotype sequences and weak evidence for purifying selection in the Zp2 coding haplotype sequences. Contrary to a general genetic pattern that genes or genomic regions contributing to reproductive isolation between species often evolve rapidly and show little or no gene flow between species, these results demonstrate that, particularly, those sequenced exons of the Zp2 and the Zp3 did not show any contribution to reproductive isolation between the bovine species studied here.

Reproductive protein evolution in two cryptic species of marine chordate

Background

Reproductive character displacement (RCD) is a common and taxonomically widespread pattern. In marine broadcast spawning organisms, behavioral and mechanical isolation are absent and prezygotic barriers between species often operate only during the fertilization process. Such barriers are usually a consequence of differences in the way in which sperm and egg proteins interact, so RCD can be manifest as faster evolution of these proteins between species in sympatry than allopatry. Rapid evolution of these proteins often appears to be a consequence of positive (directional) selection. Here, we identify a set of candidate gamete recognition proteins (GRPs) in the ascidian Ciona intestinalis and showed that these GRPs evolve more rapidly than control proteins (those not involved in gamete recognition). Choosing a subset of these gamete recognition proteins that show evidence of positive selection (CIPRO37.40.1, CIPRO60.5.1, CIPRO100.7.1), we then directly test the RCD hypothesis by comparing divergence (omega) and polymorphism (McDonald-Kreitman, Tajima's D, Fu and Li's D and F, Fay and Wu's H) statistics in sympatric and allopatric populations of two distinct forms of C. intestinalis (Types A and B) between which there are strong post-zygotic barriers.

Results

Candidate gamete recognition proteins from two lineages of C. intestinalis (Type A and B) are evolving more rapidly than control proteins, consistent with patterns seen in insects and mammals. However, ω (d_N/d_S) is not significantly different between the sympatric and allopatric populations, and none of the polymorphism statistics show significant differences between sympatric and allopatric populations.

Conclusions

Enhanced prezygotic isolation in sympatry has become a well-known feature of gamete recognition proteins in marine broadcast spawners. But in most cases the evolutionary process or processes responsible for this pattern have not been identified. Although gamete recognition proteins in C. intestinalis do appear to evolve more rapidly, on average, than proteins with other functions, rates of evolution are not different in allopatric and sympatric populations of the two reproductively isolated forms. That sympatry is probably human-mediated, and therefore recent, may explain the absence of RCD.

Cryptic female preference for genetically unrelated males is mediated by ovarian fluid in the guppy

As inbreeding is costly, it has been suggested that polyandry may evolve as a means to reduce the negative fitness consequences of mating with genetically related males. While several studies provide support for this hypothesis, evidence of pure post-copulatory mechanisms capable of biasing paternity towards genetically unrelated males is still lacking; yet these are necessary to support inbreeding avoidance models of polyandry evolution. Here we showed, by artificially inseminating a group of female guppies with an equal number of sperm from related (full-sib) and unrelated males, that sperm competition success of the former was 10 per cent lower, on average, than that of the unrelated male. The paternity bias towards unrelated males was not due to differential embryo survival, as the size of the brood produced by control females, which were artificially inseminated with the sperm of a single male, was not influenced by their relatedness with the male. Finally, we collected ovarian fluid (OF) from virgin females. Using computer-assisted sperm analysis, we found that sperm velocity, a predictor of sperm competition success in the guppy, was significantly lower when measured in a solution containing the OF from a sister as compared with that from an unrelated female. Our results suggest that sperm-OF interaction mediates sperm competition bias towards unrelated mates and highlight the role of post-copulatory mechanisms in reducing the cost of mating with relatives in polyandrous females.

Diversity-enhancing selection acts on a female reproductive protease family in four subspecies of Drosophila mojavensis

Protein components of the Drosophila male ejaculate are critical modulators of reproductive success, several of which are known to evolve rapidly. Recent evidence of adaptive evolution in female reproductive tract proteins suggests this pattern may reflect sexual selection at the molecular level. Here we explore the evolutionary dynamics of a five-paralog gene family of female reproductive proteases within geographically isolated subspecies of Drosophila mojavensis. Remarkably, four of five paralogs show exceptionally low differentiation between subspecies and unusually structured haplotypes that suggest the retention of old polymorphisms. These gene genealogies are accompanied by deviations from neutrality consistent with diversifying selection. While diversifying selection has been observed among the reproductive molecules of mammals and marine invertebrates, our study provides the first evidence of this selective regime in any Drosophila reproductive protein, male or female.

KIFC1-like motor protein associates with the cephalopod manchette and participates in sperm nuclear morphogenesis in Octopus tankahkeei

Background

Nuclear morphogenesis is one of the most fundamental cellular transformations taking place during spermatogenesis. In rodents, a microtubule-based perinuclear structure, the manchette, and a C-terminal kinesin motor KIFC1 are believed to play crucial roles in this process. Spermatogenesis in Octopus tankahkeei is a good model system to explore whether evolution has created a cephalopod prototype of mammalian manchette-based and KIFC1-dependent sperm nuclear shaping machinery.

Methodology/Principal Findings

We detected the presence of a KIFC1-like protein in the testis, muscle, and liver of O. tankahkeei by Western Blot. Then we tracked its dynamic localization in spermatic cells at various stages using Immunofluorescence and Immunogold Electron Microscopy. The KIFC1-like protein was not expressed at early stages of spermatogenesis when no significant morphological changes occur, began to be present in early spermatid, localized around and in the nucleus of intermediate and late spermatids where the nucleus was dramatically elongated and compressed, and concentrated at one end of final spermatid. Furthermore, distribution of the motor protein during nuclear elongation and condensation overlapped with that of the cephalopod counterpart of manchette at a significant level.

Conclusions/Significance

The results support the assumption that the protein is actively involved in sperm nuclear morphogenesis in O. tankahkeei possibly through bridging the manchette-like perinuclear microtubules to the nucleus and assisting in the nucleocytoplasmic trafficking of specific cargoes. This study represents the first description of the role of a motor protein in sperm nuclear shaping in cephalopod.

Statistical properties of the branch-site test of positive selection

The branch-site test is a likelihood ratio test to detect positive selection along prespecified lineages on a phylogeny that affects only a subset of codons in a protein-coding gene, with positive selection indicated by accelerated nonsynonymous substitutions (with ω = d(N)/d(S) > 1). This test may have more power than earlier methods, which average nucleotide substitution rates over sites in the protein and/or over branches on the tree. However, a few recent studies questioned the statistical basis of the test and claimed that the test generated too many false positives. In this paper, we examine the null distribution of the test and conduct a computer simulation to examine the false-positive rate and the power of the test. The results suggest that the asymptotic theory is reliable for typical data sets, and indeed in our simulations, the large-sample null distribution was reliable with as few as 20-50 codons in the alignment. We examined the impact of sequence length, the strength of positive selection, and the proportion of sites under positive selection on the power of the branch-site test. We found that the test was far more powerful in detecting episodic positive selection than branch-based tests, which average substitution rates over all codons in the gene and thus miss the signal when most codons are under strong selective constraint. Recent claims of statistical problems with the branch-site test are due to misinterpretations of simulation results. Our results, as well as previous simulation studies that have demonstrated the robustness of the test, suggest that the branch-site test may be a useful tool for detecting episodic positive selection and for generating biological hypotheses for mutation studies and functional analyses. The test is sensitive to sequence and alignment errors and caution should be exercised concerning its use when data quality is in doubt.

Species recognition and patterns of population variation in the reproductive structures of a damselfly genus

The selection pressures imposed by mate choice for species identity should impose strong stabilizing selection on traits that confer species identity to mates. Thus, we expect that such traits should show nonoverlapping distributions among closely related species, but show little to no variance among populations within a species. We tested these predictions by comparing levels of population differentiation in the sizes and shapes of male cerci (i.e., the clasper structures used for species identity during mating) of six Enallagma damselfly species. Cerci shapes were nonoverlapping among Enallagma species, and five of six Enallagma species showed no population variation across their entire species ranges. In contrast, cerci sizes overlapped among species and varied substantially among populations within species. These results, taken with previous studies, suggest that cerci shape is a primary feature used in species recognition used to discriminate conspecific from heterospecifics during mating.

Differential fertilization success between two populations of eastern oyster, Crassostrea virginica

Identification of mechanisms promoting prezygotic reproductive isolation and their prevalence are key goals in evolutionary biology because of their potential role in speciation. In marine broadcast-spawning species, molecular interactions between gamete surface proteins are more important than mating behavior for determining reproductive compatibility. Evidence for differential fertilization capacity has been reported from experiments utilizing competing sperm from two males sampled within populations and between species, but to our knowledge conspecific populations that might have diverged in allopatry have never been tested on the basis of sperm competition. In the present study, the gametic compatibility and embryo survivorship from matings between two allopatric populations of Crassostrea virginica, the eastern oyster, on either side of a genetic step cline were investigated. Fertilization success, embryo survival, and paternity data all indicated an absence of strong reproductive barriers between the two oyster populations, implicating other mechanisms for maintenance of the cline step. Sperm from northern male oysters showed a tendency to produce more larvae than expected when competing with sperm from southern male oysters. Although the northern male advantage was not strong, the trend implies that long-distance dispersal across the step cline might more successfully result in north-to-south gene flow than the reverse, providing a mechanistic hypothesis explaining the asymmetric cline shape.

Cloning and sequence analysis of Bufo arenarum oviductin cDNA and detection of its orthologous gene expression in the mouse female reproductive tract

The glycoprotein envelope surrounding the Bufo arenarum egg exists in different functional forms. Conversion between types involves proteolysis of specific envelope glycoproteins. When the egg is released from the ovary, the envelope cannot be penetrated by sperm. Conversion to a penetrable state occurs during passage through the pars recta portion of the oviduct, where oviductin, a serine protease with trypsin-like substrate specificity, hydrolyzes two kinds of envelope glycoproteins: gp84 and gp55. The nucleotide sequence of a 3203 bp B. arenarum oviductin cDNA was obtained. Deduced amino acid sequence showed a complete open reading frame encoding 980 amino acids. B. arenarum oviductin is a multi-domain protein with a protease domain at the N-terminal region followed by two CUB domains and toward the C-terminal region another protease domain, which lacked an active histidine site, and one CUB domain. Expression of ovochymase 2, the mammalian orthologous of amphibian oviductin, was assayed in mouse female reproductive tract. Ovochymase 2 mRNA was unnoticeable in the mouse oviduct but expression was remarkable in the uterus. Phylogenetic relationship between oviductin and ovochymase 2 opens the possibility to understand the role of this enzyme in mammalian reproduction.

Extreme positive selection on a new highly-expressed larval glycoprotein (LGP) gene in Galaxias fishes (Osmeriformes: Galaxiidae)

We describe the intron-exon structure and DNA/protein sequences of a new larval glycoprotein (LGP) gene from nine species of galaxiid fish. The gene has a distant similarity to Danio THP (Tamm-Horsfall urinary glycoprotein; uromodulin) and cichlid SPP120 (seminal plasma glycoprotein) due to conserved features of its zona pellucida (ZP) domain, including eight highly conserved cysteines and a consensus furin cleavage site. Using a combination of 454 sequencing of cDNA and exon-primed intron-spanning sequencing of genomic DNA, we obtained full sequences of the coding region (996 bp) and its intervening sequences (1,459 bp). LGP shows an exceptionally strong signal of positive selection over the entire coding region, as evidenced by d(N)/d(S) values >1. Across nine species of Galaxias, 87/332 (26%) amino acid residues are variable, compared with 9/386 (2%) for mitochondrial cytochrome b (cytb) in the same group of species. Across 36 interspecific pairwise comparisons, genetic distances are in all cases larger for coding region than for introns, by a factor of 2.4-fold on average. Reading frame, gene structure, splice sites, and many ZP motifs are conserved across all species. Together with the fact that the gene is expressed in all species, these results argue clearly against the possibility of a pseudogene. We show by 454 sequencing and quantitative polymerase chain reaction that the transcript is abundant (ca. 0.5%) in newly hatched larvae and appears to be almost absent from a range of adult tissues. We postulate that the strong Darwinian evolution exhibited by this protein may reflect some type of immunoprotection at this vulnerable larval stage.

A likelihood ratio test of speciation with gene flow using genomic sequence data

Genomic sequence data may be used to test hypotheses about the process of species formation. In this paper, I implement a likelihood ratio test of variable species divergence times over the genome, which may be considered a test of the null model of allopatric speciation without gene flow against the alternative model of parapatric speciation with gene flow. Two models are implemented in the likelihood framework, which accommodate coalescent events in the ancestral populations in a phylogeny of three species. One model assumes a constant species divergence time over the genome, whereas another allows it to vary. Computer simulation shows that the test has acceptable false positive rate but to achieve reasonable power, hundreds or even thousands of genomic loci may be necessary. The test is applied to genomic data from the human, chimpanzee, and gorilla.

Incomplete reproductive isolation in the blue mussel (Mytilus edulis and M. trossulus) hybrid zone in the Northwest Atlantic: role of gamete interactions and larval viability

Reproductive isolation in free-spawning organisms may involve only small changes in the gamete surface molecules that control fertilization, linking gamete incompatibility and speciation. Most studies have focused on species in which natural hybrids are absent and reproductive isolation is complete, but how gamete incompatibility evolves remains unclear. Reproductive isolation is incomplete between two sympatric mussel species (Mytilus edulis, M. trossulus) that hybridize in nature. In this study prezygotic and postzygotic components of reproductive incompatibility were examined in laboratory crosses. Conspecific crosses showed significantly greater rates of fertilization than heterospecific crosses, although some females of both species showed heterospecific gamete compatibility. The proportion of fertilized eggs developing into normal larvae was not significantly different between conspecific and heterospecific crosses, but survival of normal larvae was greater for conspecific crosses. Mixed-species sperm experiments suggested that conspecific sperm preference may further limit hybridization. The different components of reproductive incompatibility and total incompatibility varied among females of both species. Although our study has shown that partial reproductive isolation between M. edulis and M. trossulus involves both prezygotic gamete interactions and postzygotic larval survival, further research is required to determine the potential role of gamete incompatibility in the evolution of complete reproductive isolation between these species.

Combined EST and proteomic analysis identifies rapidly evolving seminal fluid proteins in Heliconius butterflies

Seminal fluid proteins (SFPs) directly influence a wide range of reproductive processes, including fertilization, sperm storage, egg production, and immune response. Like many other reproductive proteins, the molecular evolution of SFPs is generally characterized by rapid and frequently adaptive evolution. However, the evolutionary processes underlying this often-documented pattern have not yet been confidently determined. A robust understanding of the processes governing SFP evolution will ultimately require identifying SFPs and characterizing their evolution in many different taxa, often where only limited genomic resources are available. Here, we report the first comprehensive molecular genetic and evolutionary analysis of SFPs conducted in Lepidoptera (moths and butterflies). We have identified 51 novel SFPs from two species of Heliconius butterflies (Heliconius erato and Heliconius melpomene) by combining "indirect" bioinformatic and expression analyses of expressed sequence tags from male accessory gland and wing tissues with "direct" proteomic analyses of spermatophores. Proteomic analyses identified fewer SFPs than the indirect criteria but gave consistent results. Of 51 SFPs, 40 were identified in both species but fewer than half could be functionally annotated via similarity searches (Blast, IPRscan, etc.). The majority of annotated Heliconius SFPs were predicted to be chymotrypsins. Comparisons of Heliconius SFPs with those from fruit fly, mosquito, honeybee, and cricket suggest that gene turnover is high among these proteins and that SFPs are rarely conserved across insect orders. Pairwise estimates of evolutionary rates between SFPs and nonreproductive proteins show that, on average, Heliconius SFPs are evolving rapidly. At least one of these SFPs is evolving adaptively (dN/dS > 1), implicating a role for positive selection in this rapid evolution. This work establishes a strong precedent for future research on the causes and consequences of reproductive protein evolution in the Lepidoptera. Butterflies and moths have an extremely rich history of organismal research, which will provide an informative ecological context for further molecular evolutionary investigations.

Proteomics enhances evolutionary and functional analysis of reproductive proteins

Reproductive proteins maintain species-specific barriers to fertilization, affect the outcome of sperm competition, mediate reproductive conflicts between the sexes, and potentially contribute to the formation of new species. However, the specific proteins and molecular mechanisms that underlie these processes are understood in only a handful of cases. Advances in genomic and proteomic technologies enable the identification of large suites of reproductive proteins, making it possible to dissect reproductive phenotypes at the molecular level. We first review these technological advances and describe how reproductive proteins are identified in diverse animal taxa. We then discuss the dynamic evolution of reproductive proteins and the potential selective forces that act on them. Finally, we describe molecular and genomic tools for functional analysis and detail how evolutionary data may be used to make predictions about interactions among reproductive proteins.

Simultaneous positive and negative frequency-dependent selection on sperm bindin, a gamete recognition protein in the sea urchin Strongylocentrotus purpuratus

Gamete-recognition proteins often, but not always, evolve rapidly. We explored how variation in sperm bindin influences reproductive success of the sea urchin Strongylocentrotus purpuratus during group spawning in the sea. Despite large variation in male and female abundance and neighbor distances, males with common genotypes had higher reproductive success than males with rare genotypes. However, males with a relatively uncommon proline-for-serine substitution were the most successful. Females also showed a fitness consequence of sperm-bindin genotype, suggesting linkage disequilibrium between the sperm-bindin locus and the egg receptor locus. Females with common genotypes had higher reproductive success than rare genotypes, but females with relatively uncommon insertions were most successful. Overall, these results suggest that rare male proteins are selected against, as supported by molecular evidence of purifying selection and probably caused by poor matches to the female receptor protein. Within the pool of moderately common to common alleles, however, individuals with less-common functional variants were favored and probably maintained by negative frequency-dependent selection. These results support the hypothesis that sperm availability and sexual conflict influence the evolution of gamete recognition systems in broadcast spawners and highlight the benefits of combining fitness measures with molecular signatures for estimation of patterns of selection.

Proteomic discovery of previously unannotated, rapidly evolving seminal fluid genes in Drosophila

As genomic sequences become easier to acquire, shotgun proteomics will play an increasingly important role in genome annotation. With proteomics, researchers can confirm and revise existing genome annotations and discover completely new genes. Proteomic-based de novo gene discovery should be especially useful for sets of genes with characteristics that make them difficult to predict with gene-finding algorithms. Here, we report the proteomic discovery of 19 previously unannotated genes encoding seminal fluid proteins (Sfps) that are transferred from males to females during mating in Drosophila. Using bioinformatics, we detected putative orthologs of these genes, as well as 19 others detected by the same method in a previous study, across several related species. Gene expression analysis revealed that nearly all predicted orthologs are transcribed and that most are expressed in a male-specific or male-biased manner. We suggest several reasons why these genes escaped computational prediction. Like annotated Sfps, many of these new proteins show a pattern of adaptive evolution, consistent with their potential role in influencing male sperm competitive ability. However, in contrast to annotated Sfps, these new genes are shorter, have a higher rate of nonsynonymous substitution, and have a markedly lower GC content in coding regions. Our data demonstrate the utility of applying proteomic gene discovery methods to a specific biological process and provide a more complete picture of the molecules that are critical to reproductive success in Drosophila.

The evolution of postpollination reproductive isolation in Costus

Reproductive isolation is critical to the diversification of species. Postpollination barriers may be important in limiting gene flow between closely related species, but they are relatively cryptic and their evolution is poorly understood. Here, we review the role of postpollination reproductive isolation in plants, including the various stages at which it operates and the hypotheses for how it may evolve. We then review empirical studies in the plant genus Costus, evaluating documented postpollination barriers in light of these hypotheses. We summarize isolation due to parental style length differences and present evidence supporting the hypothesis that the differences are in part a by-product of selection on floral morphology. Additionally, we show that reduced pollen adhesion, germination, and tube growth contribute to reproductive isolation between two closely related sympatric species of Costus. Geographic variation in the strength of these crossing barriers supports the hypothesis that they evolved under reinforcement, or direct natural selection to strengthen isolation.

Evolution of sex-dependent gene expression in three recently diverged species of Drosophila

Sexual dimorphism in morphological, physiological, and behavioral traits is pervasive in animals, as is the observation of strong sexual dimorphism in genomewide patterns of gene expression in the few species where this has been studied. Studies of transcriptome divergence show that most interspecific transcriptional divergence is highly sex dependent, an observation consistent with the action of sex-dependent natural selection during species divergence. However, few transcriptome evolution studies have been conducted between recently diverged species (<1 MY). Here, we present analyses of sex-biased transcriptome divergence in sexually mature adults of three recently diverged species of Drosophila: Drosophila pseudoobscura, D. persimilis, and D. pseudoobscura bogotana. Data were collected using a custom designed Agilent oligonucleotide. Expression was detected in 12,507 genes. About 80% of the expressed genes show sex-biased expression in each species. Across species, 21% of the transcriptome shows switches between nonsex bias and sex bias, and just 0.9% of the transcriptome shows reversals of sex-biased expression. Over 80% of the expression divergence between species is due to changes in one sex only. About 15% of the expression divergence between species is due to changes in the same direction in both sexes and just 2% is due to changes in both sexes but in opposite directions. In agreement with previous studies, we observe a high level of sex-dependent transcriptome divergence and strong demasculinization of the two arms of the X chromosome in all species. However, in contrast to previous studies we find that male-biased genes do not have higher levels of expression divergence than non-sex-biased genes, and sex-biased genes show higher levels of expression divergence in the alternate sex, suggesting that sex-biased genes endure stronger selection when expressed in the alternate sex.

Comparative and functional genomics of lipases in holometabolous insects

Lipases have key roles in insect lipid acquisition, storage and mobilisation and are also fundamental to many physiological processes underpinning insect reproduction, development, defence from pathogens and oxidative stress, and pheromone signalling. We have screened the recently sequenced genomes of five species from four orders of holometabolous insects, the dipterans Drosophila melanogaster and Anopheles gambiae, the hymenopteran Apis mellifera, the moth Bombyx mori and the beetle Tribolium castaneum, for the six major lipase families that are also found in other organisms. The two most numerous families in the insects, the neutral and acid lipases, are also the main families in mammals, albeit not in Caenorhabditis elegans, plants or microbes. Total numbers of the lipases vary two-fold across the five insect species, from numbers similar to those in mammals up to numbers comparable to those seen in C. elegans. Whilst there is a high degree of orthology with mammalian lipases in the other four families, the great majority of the insect neutral and acid lipases have arisen since the insect orders themselves diverged. Intriguingly, about 10% of the insect neutral and acid lipases have lost motifs critical for catalytic function. Examination of the length of lid and loop regions of the neutral lipase sequences suggest that most of the insect lipases lack triacylglycerol (TAG) hydrolysis activity, although the acid lipases all have intact cap domains required for TAG hydrolysis. We have also reviewed the sequence databases and scientific literature for insights into the expression profiles and functions of the insect neutral and acid lipases and the orthologues of the mammalian adipose triglyceride lipase which has a pivotal role in lipid mobilisation. These data suggest that some of the acid and neutral lipase diversity may be due to a requirement for rapid accumulation of dietary lipids. The different roles required of lipases at the four discrete life stages of holometabolous insects may also contribute to the diversity of lipases required by insects. In addition, insects use lipases to perform roles for which there are no correlates in mammals, including as yolk and male accessory gland proteins.

Intralocus sexual conflict

Intralocus sexual conflict arises when there are sex-specific optima for a trait that is expressed in both sexes and when the constraint of a shared gene pool prevents males and females from reaching their optima independently. This situation may result in a negative intersexual correlation for fitness. Here I first discuss key differences between intra- and interlocus conflict, the type of sexual conflict that arises in mating interactions between males and females. I then review the experimental evidence for the existence of genomewide sexually antagonistic variation and discuss how intralocus conflict can be resolved. Substantial genomewide sexually antagonistic variation exists in Drosophila melanogaster lab populations. Yet, in the same species, sex-specific gene regulation appears to evolve rapidly, suggesting that the obstacles to the resolution of intralocus conflict are minor. The fact that negative intersexual correlations for fitness are observed even if sexual dimorphism can evolve rapidly suggests that intralocus conflict is highly dynamic. The final part of this review examines the evolutionary consequences of intralocus sexual conflict for the evolution of the sex chromosomes, sexual selection, and sex determination. Intralocus conflict helps to explain many of the peculiar features of the sex chromosomes and has shaped the functional bias and expression biases of sex-linked genes. The genomic distribution of sexually selected genes, in particular, affects sexual selection in various ways. The presence of sexually antagonistic variation can strongly interfere with the good genes' process of sexual selection and erode the genetic benefits of mate choice. Regarding sex determination, this review concentrates on evolutionary transitions between different sex determination mechanisms. Such transitions have occurred frequently in several taxa. Theory and empirical data suggest an important role for intralocus conflict in triggering switches between sex determination systems.

Endogenous signaling pathways and chemical communication between sperm and egg

Red abalone (Haliotis rufescens) sperm detect a waterborne chemical cue released by conspecific eggs, and change their swimming behavior to increase the likelihood of fertilization success. Previously, we isolated the natural sperm attractant by bioassay-guided fractionation and high-performance liquid chromatography, and chemically identified it as the free-amino acid l-tryptophan (l-Trp). In the present study, levels of this ecologically meaningful compound were quantified in various abalone tissues, and in freshly spawned eggs. Tryptophan was the least abundant of 19 dissolved free amino acids (DFAAs) in ovary, testis, foot muscle, gill, stomach and hemolymph. As a proportion of the DFAA pool, however, Trp concentrations were significantly elevated in eggs (three- to seven-times higher) relative to all other sampled tissues. Natural rates of Trp release from eggs also were measured and correlated with fertility. Fertilization success peaked during an initial 30 min period (post-spawn), but decreased to nil over the next 50 min. Closely paralleling these events, Trp accumulated in seawater around freshly spawned eggs for the first 45 min (post-spawn) before decaying rapidly from solution. Older eggs stopped releasing Trp approximately when they became infertile, revealing a critical link between gamete physiology and chemical signaling. This apparent negative feedback loop did not arise from tryptophan oxidation, uptake by bacteria in seawater, or a degrading enzyme released by eggs. As a metabolic precursor critical to development of the larval nervous system, Trp could be an honest indicator of egg fitness for prospective sperm suitors. Our results suggest that endogenous signaling pathways have been co-opted for external communication between gametes, as an adaptation to increase reproductive success by promoting sperm navigation towards fertile eggs.

Speciation and the evolution of gamete recognition genes: pattern and process

Proteins on gamete surfaces are major determinants of fertilization success, particularly in free-spawning animals. Molecular analyses of these simple genetic systems show rapid evolution, positive selection, accelerated coalescence and, sometimes, extensive polymorphism. Careful analysis of the behavior of sperm produced by males with different gamete alleles shows that these alleles can deliver significant functional differences. Three forms of allele-specific fertilization advantage have been shown: assortative mating based on gamete type, rare allele advantage and heterozygote superiority. Models suggest that sperm and egg proteins may be coevolutionary partners that can alternate between directional selection for high fertilization ability and cyclic adaptation of eggs and sperm driven by sexual conflict. These processes act within allopatric populations and may accelerate their divergence if gamete adaptations in separate demes reduce cross-fertilization. Reproductive character displacement by reinforcement may play a diversifying role when previously allopatric populations rejoin. In circumstance that might prove to be common, divergence in sympatry can be driven by sexual conflict or by association of mating types with ecological differences. The ecology of fertilization, especially the degree of sperm competition and egg death via polyspermy, are important determinants of the strength and direction of selection on gametes. Free-spawning animals allow careful analysis of gamete recognition -from the behavior of adults and interactions of gametes, to molecular patterns of allele divergence. Future research efforts on the evolutionary consequences of fertilization ecology, and the interaction between extensive variation in egg surface proteins and sperm fertilization ability, are particularly needed.

The benefits of male ejaculate sex peptide transfer in Drosophila melanogaster

The accessory gland protein (Acp) ejaculate molecules of male Drosophila melanogaster mediate sexual selection and sexual conflict at the molecular level. However, to date no studies have comprehensively measured the timing and magnitude of fitness benefits to males of transferring specific Acps. This is an important omission because without this information it is not possible to fully understand the strength and form of selection acting on adaptations such as Acps. Here, we measured the fitness benefits to males of ejaculate sex peptide (SP) transfer. SP is of interest because it is a candidate for mediating sexual conflict: its frequent receipt reduces female fitness. In single matings with virgin females SP is known to increase egg laying and decrease receptivity. Hence, we predicted that SP could: (i) boost a male's absolute paternity by increasing offspring production and delaying female remating and/or (ii) boost relative paternity share. We tested these predictions using two different lines of SP-lacking males, in both two-mating and free-mating assay conditions. SP transfer conferred higher absolute, but not relative, male reproductive success. In matings with virgin females, SP transfer increased mating productivity and delayed remating and hence the onset of sperm competition. In already mated females, SP transfer did not elevate absolute progeny production, but did increase intermating intervals and hence the period over which a male could gain paternity. Consistent with this, under free-mating conditions over an extended period, we detected a 'per-mating' fitness benefit for males transferring SP. These benefits are consistent with a role for SP in mediating conflict, with SP acting to maximize short-term fitness benefits for males.

Adaptive evolution of gamete-recognition proteins in birds

Gamete-recognition proteins have been shown to evolve by positive selection in diverse organism groups, such as marine invertebrates and mammals, although underlying evolutionary mechanisms driving this rapid divergence are poorly understood. However, several hypotheses have been put forward to explain the observed pattern, including different forms of sexual conflict and sperm competition. Because female gametes require more energy to produce than male gametes, female organisms suffer more when fertilisation goes wrong. One process that results in a failed mammalian fertilisation is polyspermy, when >1 sperm fertilises the egg. However in birds, there is no such sexual conflict because multiple sperm typically bind and fuse with the egg. If sexual conflict driven by polyspermy avoidance is important for the evolution of gamete-recognition proteins in vertebrates, we expect to find positive selection in the genes to be less pronounced in birds. We therefore sequenced six genes (ZP1, ZP2, ZP4, ZPAX, CD9, and Acrosin) encoding gamete-recognition proteins in several bird species to test for positive selection. For comparison, we also analysed ortologous sequences in a set of mammalian species. We found no major differences in the occurrence of adaptive evolution and the strength of selection between bird and mammal orthologs. From this we conclude that polyspermy avoidance does not act as the main underlying evolutionary force shaping the rate of evolution in these genes. We discuss other possible processes that could explain positive selection of gamete-recognition proteins in birds and mammals, such as hybridisation avoidance, cryptic female choice, and postcopulatory sperm competition.

Gamete recognition in higher plants: an abstruse but charming mystery

Although much effort has been made to uncover the mechanism underlying double fertilization, little knowledge has been acquired for understanding the molecular base of gamete recognition, mainly because of technical limitations. Still, progress has been made in terms of the mechanism, including the identification of candidate molecules that are involved in gamete recognition in angiosperms. New cues for gamete recognition have been found by the successful separation of the gametes and construction of gamete-specific cDNA libraries in several species, and the application of molecular approaches for studying this process by mutations. Thus, the topic is considered an abstruse but charming mystery.

Post-mating gene expression profiles of female Drosophila melanogaster in response to time and to four male accessory gland proteins

In Drosophila melanogaster, the genetic and molecular bases of post-mating changes in the female's behavior and physiology are poorly understood. However, DNA microarray studies have demonstrated that, shortly after mating, transcript abundance of >1700 genes is altered in the female's reproductive tract as well as in other tissues. Many of these changes are elicited by sperm and seminal fluid proteins (Acps) that males transfer to females. To further dissect the transcript-level changes that occur following mating, we examined gene expression profiles of whole female flies at four time points following copulation. We found that, soon after copulation ends, a large number of small-magnitude transcriptional changes occurred in the mated female. At later time points, larger magnitude changes were seen, although these occurred in a smaller number of genes. We then explored how four individual Acps (ovulin, Acp36DE, Acp29AB, and Acp62F) with unique functions independently affected gene expression in females shortly after mating. Consistent with their early and possibly local action within the female, ovulin and Acp36DE caused relatively few gene expression changes in whole bodies of mated females. In contrast, Acp29AB and Acp62F modulated a large number of transcriptional changes shortly after mating.

Reproductive protein evolution within and between species: maintenance of divergent ZP3 alleles in Peromyscus

In a variety of animal taxa, proteins involved in reproduction evolve more rapidly than nonreproductive proteins. Most studies of reproductive protein evolution, however, focus on divergence between species, and little is known about differentiation among populations within a species. Here we investigate the molecular population genetics of the protein ZP3 within two Peromyscus species. ZP3 is an egg coat protein involved in primary binding of egg and sperm and is essential for fertilization. We find that amino acid polymorphism in the sperm-combining region of ZP3 is high relative to silent polymorphism in both species of Peromyscus. In addition, while there is geographical structure at a mitochondrial gene (Cytb), a nuclear gene (Lcat) and eight microsatellite loci, we find no evidence for geographical structure at Zp3 in Peromyscus truei. These patterns are consistent with the maintenance of ZP3 alleles by balancing selection, possibly due to sexual conflict or pathogen resistance. However, we do not find evidence that reinforcement promotes ZP3 diversification; allelic variation in P. truei is similar among populations, including populations allopatric and sympatric with sibling species. In fact, most alleles are present in all populations sampled across P. truei's range. While additional data are needed to identify the precise evolutionary forces responsible for sequence variation in ZP3, our results suggest that in Peromyscus, selection to maintain divergent alleles within species contributes to the pattern of rapid amino acid substitution observed among species.

Detecting the Genetic Signature of Natural Selection in Human Populations: Models, Methods, and Data

Patterns of DNA sequence variation in the genome contain a record of past selective events. The ability to collect increasingly large data sets of polymorphisms has allowed investigators to perform hypothesis-driven studies of candidate genes as well as genome-wide scans for signatures of adaptations. This genetic approach to the study of natural selection has identified many signals consistent with predictions from anthropological studies. Selective pressures related to variation in climate, diet, and pathogen exposure have left strong marks on patterns of human variation. Additional signals of adaptations are observed in genes involved in chemosensory perception and reproduction. Several ongoing projects aim to sequence the complete genome of 1000 individuals from different human populations. These large-scale projects will provide data for more complete genome scans of selection, but more focused studies aimed at testing specific hypotheses will continue to hold an important place in elucidating the history of adaptations in humans.

Adaptive gamete-recognition divergence in a hybridizing Mytilus population

Gamete-recognition proteins often evolve rapidly, but it is not known if their divergence occurs within species and corresponds with the evolution of reproductive isolation, or if divergence typically accumulates between already isolated lineages. We examined the evolution of a candidate gamete-recognition protein in several sympatric and allopatric populations of Mytilus blue mussels, species that hybridize in nature. Within a single species, Mytilus galloprovincialis, we found adaptive divergence of Lysin-M7, a sperm acrosomal protein that dissolves the egg vitelline envelope during fertilization. Mytilus galloprovincialis Lysin-M7 alleles group into two distinct clades (termed G and G(D)), and individual alleles in these clades are separated from each other by at least three and up to eleven amino-acid substitutions. Maximum-likelihood estimates of selective pressure (dN/dS =omega) implicate selection in the divergence between M. galloprovincialis Lysin-M7 clades, and within the G(D) clade. Exact tests of population differentiation indicate that the relative frequency of G and G(D) Lysin-M7 alleles differs significantly among M. galloprovincialis populations. Compared with allopatric Mediterranean samples, Lysin-M7 alleles in the G(D) clade are found at elevated frequency in samples from the East Atlantic and California, areas of secondary contact and hybridization between Mytilus species, and Australia, an area of unknown species composition. Adaptive divergence between the alleles most common in allopatry and those found at elevated frequency in samples from sympatry suggests that selection pressures acting in hybridizing populations, likely following Pleistocene secondary contact with M. edulis in the East Atlantic, drove the divergence of Lysin-M7 in M. galloprovincialis.

Adaptive evolution in an avian reproductive protein: ZP3

Proteins involved in reproduction appear to be evolving adaptively across taxa. This rapid evolution is thought to be the result of forces involved in sexual selection. One of the most often suggested of these forces is sexual conflict involving sperm competition and polyspermy avoidance. Bird species offer a unique opportunity to test this hypothesis since the avian egg coat tolerates physiological polyspermy, or the penetration of multiple sperm during fertilization, without negative effects on later development. Despite this, and the extensive amount of data gathered on sexual selection in birds, there are limited studies on the patterns of evolution of avian reproductive proteins. Here we present an analysis of the pattern of evolution of Zona Pellucida 3 (ZP3), a protein present on the avian egg coat. We found that, across several galliform and a single anseriform species, ZP3 appears to be diverging by positive adaptive evolution. In an exploratory analysis of portions of the gene in Callipepla californica we also found evidence of a selective sweep at the putative sperm binding region of the protein. In sum, ZP3 in birds, like reproductive proteins in other species, appears to be adaptively evolving. This result suggests that polyspermy avoidance is not sufficient to explain positive Darwinian selection in reproductive proteins across taxonomic groups. Clearly, the inclusion of bird species in the study of reproductive proteins across taxa promises to add greatly to the discussion of the factors driving the widespread phenomenon of adaptive evolution in reproductive proteins.

THE RISK OF POLYSPERMY IN THREE CONGENERIC SEA URCHINS AND ITS IMPLICATIONS FOR GAMETIC INCOMPATIBILITY AND REPRODUCTIVE ISOLATION

Developmental failure caused by excess sperm (polyspermy) is thought to be an important mechanism driving the evolution of gamete-recognition proteins, reproductive isolation, and speciation in marine organisms. However, these theories assume that there is heritable variation in the susceptibility to polyspermy and that this variation is related to the overall affinity between sperm and eggs. These assumptions have not been critically examined. We investigated the relationship between ease of fertilization and susceptibility to polyspermy within and among three congeneric sea urchins. The results from laboratory studies indicate that, both within and among species, individuals and species that produce eggs capable of fertilization at relatively low sperm concentrations are more susceptible to polyspermy, whereas individuals and species producing eggs that require higher concentrations of sperm to be fertilized are more resistant to polyspermy. This relationship sets the stage for selection on gamete traits that depend on sperm availability and for sexual conflict that can influence the evolution of gamete-recognition proteins and eventually lead to reproductive isolation.

Gene expression and the evolution of phenotypic diversity in social wasps

Background

Organisms are capable of developing different phenotypes by altering the genes they express. This phenotypic plasticity provides a means for species to respond effectively to environmental conditions. One of the most dramatic examples of phenotypic plasticity occurs in the highly social hymenopteran insects (ants, social bees, and social wasps), where distinct castes and sexes all arise from the same genes. To elucidate how variation in patterns of gene expression affects phenotypic variation, we conducted a study to simultaneously address the influence of developmental stage, sex, and caste on patterns of gene expression in Vespula wasps. Furthermore, we compared the patterns found in this species to those found in other taxa in order to investigate how variation in gene expression leads to phenotypic evolution.

Results

We constructed 11 different cDNA libraries derived from various developmental stages and castes of Vespula squamosa. Comparisons of overall expression patterns indicated that gene-expression differences distinguishing developmental stages were greater than expression differences differentiating sex or caste. Furthermore, we determined that certain sets of genes showed similar patterns of expression in the same phenotypic forms of different species. Specifically, larvae upregulated genes related to metabolism and genes possessing structural activity. Surprisingly, our data indicated that at least a few specific gene functions and at least one specific gene family are important components of caste differentiation across social insect taxa.

Conclusion

Despite research on various aspects of development originating from model systems, growth in understanding how development is related to phenotypic diversity relies on a growing literature of contrasting studies in non-model systems. In this study, we found that comparisons of patterns of gene expression with model systems highlighted areas of conserved and convergent developmental evolution across diverse taxa. Indeed, conserved biological functions across species implicated key functions related to how phenotypes are built. Finally, overall differences between social insect taxa suggest that the independent evolution of caste arose via distinct developmental trajectories.

Sperm nucleomorphogenesis in the cephalopod Sepia officinalis

Sperm nucleomorphogenesis in the cephalopod Sepia officinalis is the product of the interaction between perinuclear microtubules and condensing chromatin. This interaction occurs during spermiogenesis and is established through the nuclear membrane. As in other cephalopod species, the perinuclear microtubules are transient structures. In the case of S. officinalis, they begin to appear in the basal area of the early spermatid and progress from there, establishing contact with the external nuclear membrane and follow a defined, but not symmetric, geometry. Thus, the microtubules accumulate preferentially in one area of the nuclear membrane which we refer to here as the "dorsal zone". Later, the microtubules will be eliminated before the mature spermatid migrates to the epidydimis. The chromatin is condensed within the nucleus following a complex pattern, beginning as fibro-granular structures until forming fibres of approximately 45 nm diameter (patterning phases). From this stage on, an increase in the chemical basicity of DNA-interacting proteins is produced, and chromatin fibres coalesce together, being recruited to the dorsal zone of the membrane, where there is a higher density of microtubules. This last step (condensation phases) allows the chromatin fibres to be arranged parallel to the axis of the elongating nucleus, and more importantly, is deduced to cause a lateral compression of the nucleus. This lateral compression is in fact a recruitment of the ventral zone toward the dorsal zone, which brings about an important reduction in nuclear volume. The detailed observations which comprise this work complement previous studies of spermiogenesis of Sepia and other cephalopods, and will help to better understand the process of cellular morphology implicated in the evolution of sperm nuclear shape in this taxonomic group.