Species-specific heterochromatin prevents mitotic chromosome segregation to cause hybrid lethality in Drosophila

Wolbachia-mediated male killing is associated with defective chromatin remodeling

Male killing, induced by different bacterial taxa of maternally inherited microorganisms, resulting in highly distorted female-biased sex-ratios, is a common phenomenon among arthropods. Some strains of the endosymbiont bacteria Wolbachia have been shown to induce this phenotype in particular insect hosts. High altitude populations of Drosophila bifasciata infected with Wolbachia show selective male killing during embryonic development. However, since this was first reported, circa 60 years ago, the interaction between Wolbachia and its host has remained unclear. Herein we show that D. bifasciata male embryos display defective chromatin remodeling, improper chromatid segregation and chromosome bridging, as well as abnormal mitotic spindles and gradual loss of their centrosomes. These defects occur at different times in the early development of male embryos leading to death during early nuclear division cycles or large defective areas of the cellular blastoderm, culminating in abnormal embryos that die before eclosion. We propose that Wolbachia affects the development of male embryos by specifically targeting male chromatin remodeling and thus disturbing mitotic spindle assembly and chromosome behavior. These are the first observations that demonstrate fundamental aspects of the cytological mechanism of male killing and represent a solid base for further molecular studies of this phenomenon.

Roles of mutation and selection in speciation: from Hugo de Vries to the modern genomic era

One of the most important problems in evolutionary biology is to understand how new species are generated in nature. In the past, it was difficult to study this problem because our lifetime is too short to observe the entire process of speciation. In recent years, however, molecular and genomic techniques have been developed for identifying and studying the genes involved in speciation. Using these techniques, many investigators have already obtained new findings. At present, however, the results obtained are complex and quite confusing. We have therefore attempted to understand these findings coherently with a historical perspective and clarify the roles of mutation and natural selection in speciation. We have first indicated that the root of the currently burgeoning field of plant genomics goes back to Hugo de Vries, who proposed the mutation theory of evolution more than a century ago and that he unknowingly found the importance of polyploidy and chromosomal rearrangements in plant speciation. We have then shown that the currently popular Dobzhansky–Muller model of evolution of reproductive isolation is only one of many possible mechanisms. Some of them are Oka’s model of duplicate gene mutations, multiallelic speciation, mutation-rescue model, segregation-distorter gene model, heterochromatin-associated speciation, single-locus model, etc. The occurrence of speciation also depends on the reproductive system, population size, bottleneck effects, and environmental factors, such as temperature and day length. Some authors emphasized the importance of natural selection to speed up speciation, but mutation is crucial in speciation because reproductive barriers cannot be generated without mutations.

Chromatin evolution and molecular drive in speciation

Are there biological generalities that underlie hybrid sterility or inviability? Recently, around a dozen "speciation genes" have been identified mainly in Drosophila, and the biological functions of these genes are revealing molecular generalities. Major cases of hybrid sterility and inviability seem to result from chromatin evolution and molecular drive in speciation. Repetitive satellite DNAs within heterochromatin, especially at centromeres, evolve rapidly through molecular drive mechanisms (both meiotic and centromeric). Chromatin-binding proteins, therefore, must also evolve rapidly to maintain binding capability. As a result, chromatin binding proteins may not be able to interact with chromosomes from another species in a hybrid, causing hybrid sterility and inviability.

Fertilization and cytogenetic examination of interspecific reciprocal hybridization between the scallops, Chlamys farreri and Mimachlamys nobilis

Crossbreeding is a powerful tool for improving productivity and profitability in aquaculture. We conducted a pilot study of an artificial cross between two important cultivated scallops in China, Chlamys farreri and Mimachlamys nobilis, to test the feasibility of interspecific hybridization. Reciprocal hybridization experiments were performed using a single-pair mating strategy (M. nobilis ♀ × C. farreri ♂ and C. farreri ♀ × M. nobilis ♂). The fertilization of each pair was tracked using fluorescence staining of the gametes, and the chromosomes of the F1 hybrid larvae were examined via conventional karyotyping and genomic in situ hybridization (GISH). We observed moderate fertilization success in both interspecific crosses, although the overall fertilization was generally less rapid than that of intraspecific crosses. Conventional karyotyping showed that 70.4% of the viable F1 larvae in M. nobilis ♀ × C. farreri ♂ and 55.4% in C. farreri ♀ × M. nobilis ♂ comprised hybrid karyotypes (2n = 35 = 6m+5sm+11st+13t), and the results were further confirmed by GISH. Interestingly, we detected a few F1 from the M. nobilis ♀ × C. farreri ♂ cross that appeared to have developed gynogenetically. In addition, chromosome fragmentations, aneuploids and allopolyploids were observed in some F1 individuals. Our study presents evidence that the artificial cross between M. nobilis and C. farreri is experimentally possible. Further investigations of the potential heterosis of the viable F1 offspring at various developmental stages should be conducted to obtain a comprehensive evaluation of the feasibility of crossbreeding between these two scallop species.

Interspecific Y chromosome introgressions disrupt testis-specific gene expression and male reproductive phenotypes in Drosophila

The Drosophila Y chromosome is a degenerated, heterochromatic chromosome with few functional genes. Nonetheless, natural variation on the Y chromosome in Drosophila melanogaster has substantial trans-acting effects on the regulation of X-linked and autosomal genes. However, the contribution of Y chromosome divergence to gene expression divergence between species is unknown. In this study, we constructed a series of Y chromosome introgression lines, in which Y chromosomes from either Drosophila sechellia or Drosophila simulans are introgressed into a common D. simulans genetic background. Using these lines, we compared genome-wide gene expression and male reproductive phenotypes between heterospecific and conspecific Y chromosomes. We find significant differences in expression for 122 genes, or 2.84% of all genes analyzed. Genes down-regulated in males with heterospecific Y chromosomes are significantly biased toward testis-specific expression patterns. These same lines show reduced fecundity and sperm competitive ability. Taken together, these results imply a significant role for Y/X and Y/autosome interactions in maintaining proper expression of male-specific genes, either directly or via indirect effects on male reproductive tissue development or function.

Comment on "A test of the snowball theory for the rate of evolution of hybrid incompatibilities"

Matute et al. (Reports, 17 September 2010, p. 1518) tested the theory that the number of genes involved in hybrid incompatibility increases faster than linearly. However, the method they used is inappropriate because it detects genes that are haploinsufficient in a hybrid background but that would not contribute to lethality in wild-type hybrids, thus overestimating the frequency of hybrid inviability.

Comment on "A test of the snowball theory for the rate of evolution of hybrid incompatibilities"

Matute et al. (Reports, 17 September 2010, p. 1518) tested the theory that the number of genes involved in hybrid incompatibility increases faster than linearly. However, the method they used is inappropriate because it detects genes that are haploinsufficient in a hybrid background but that would not contribute to lethality in wild-type hybrids, thus overestimating the frequency of hybrid inviability.

The genetics of hybrid incompatibilities

Incompatibilities in interspecific hybrids, such as sterility and lethality, are widely observed causes of reproductive isolation and thus contribute to speciation. Because hybrid incompatibilities are caused by divergence in each of the hybridizing species, they also reveal genomic changes occurring on short evolutionary time scales that have functional consequences. These changes include divergence in protein-coding gene sequence, structure, and location, as well as divergence in noncoding DNAs. The most important unresolved issue is understanding the evolutionary causes of the divergence within species that in turn leads to incompatibility between species. Surprisingly, much of this divergence does not appear to be driven by ecological adaptation but may instead result from responses to purely mutational mechanisms or to internal genetic conflicts.

Gamma-tubulin is required for bipolar spindle assembly and for proper kinetochore microtubule attachments during prometaphase I in Drosophila oocytes

In many animal species the meiosis I spindle in oocytes is anastral and lacks centrosomes. Previous studies of Drosophila oocytes failed to detect the native form of the germline-specific γ-tubulin (γTub37C) in meiosis I spindles, and genetic studies have yielded conflicting data regarding the role of γTub37C in the formation of bipolar spindles at meiosis I. Our examination of living and fixed oocytes carrying either a null allele or strong missense mutation in the γtub37C gene demonstrates a role for γTub37C in the positioning of the oocyte nucleus during late prophase, as well as in the formation and maintenance of bipolar spindles in Drosophila oocytes. Prometaphase I spindles in γtub37C mutant oocytes showed wide, non-tapered spindle poles and disrupted positioning. Additionally, chromosomes failed to align properly on the spindle and showed morphological defects. The kinetochores failed to properly co-orient and often lacked proper attachments to the microtubule bundles, suggesting that γTub37C is required to stabilize kinetochore microtubule attachments in anastral spindles. Although spindle bipolarity was sometimes achieved by metaphase I in both γtub37C mutants, the resulting chromosome masses displayed highly disrupted chromosome alignment. Therefore, our data conclusively demonstrate a role for γTub37C in both the formation of the anastral meiosis I spindle and in the proper attachment of kinetochore microtubules. Finally, multispectral imaging demonstrates the presences of native γTub37C along the length of wild-type meiosis I spindles.

Proper chromosome segregation during cell division is essential. Missegregation of mitotic chromosomes leads to cell death or cancer, and chromosome missegregation during meiosis leads to miscarriage and birth defects. Cells utilize a bipolar microtubule-based structure known as the meiotic or mitotic spindle to segregate chromosomes. Because proper bipolar spindle formation is critically important for chromosome segregation, cells have many redundant mechanisms to ensure that this structure is properly formed. In most animal cells, centrosomes containing γ-tubulin protein complexes help organize and shape the bipolar spindle. Since meiosis I spindles in oocytes lack centrosomes, the mechanisms by which a meiotic bipolar spindle is assembled are not fully understood. In Drosophila oocytes it was not clear whether γ-tubulin played a role in bipolar spindle assembly or if it was even present on the meiotic spindle. We demonstrate that γ-tubulin plays vital roles in bipolar spindle formation and maintenance, as well as in aligning the chromosomes on the oocyte spindle. Additionally, we show that γ-tubulin is present on the bipolar spindle in Drosophila oocytes. More importantly, we demonstrate that γ-tubulin plays a critical role in the formation of the kinetochore microtubules that are required to properly orient chromosomes on the meiotic spindle.

Genetic dissection of a key reproductive barrier between nascent species of house mice

Reproductive isolation between species is often caused by deleterious interactions among loci in hybrids. Finding the genes involved in these incompatibilities provides insight into the mechanisms of speciation. With recently diverged subspecies, house mice provide a powerful system for understanding the genetics of reproductive isolation early in the speciation process. Although previous studies have yielded important clues about the genetics of hybrid male sterility in house mice, they have been restricted to F1 sterility or incompatibilities involving the X chromosome. To provide a more complete characterization of this key reproductive barrier, we conducted an F2 intercross between wild-derived inbred strains from two subspecies of house mice, Mus musculus musculus and Mus musculus domesticus. We identified a suite of autosomal and X-linked QTL that underlie measures of hybrid male sterility, including testis weight, sperm density, and sperm morphology. In many cases, the autosomal loci were unique to a specific sterility trait and exhibited an effect only when homozygous, underscoring the importance of examining reproductive barriers beyond the F1 generation. We also found novel two-locus incompatibilities between the M. m. musculus X chromosome and M. m. domesticus autosomal alleles. Our results reveal a complex genetic architecture for hybrid male sterility and suggest a prominent role for reproductive barriers in advanced generations in maintaining subspecies integrity in house mice.

Interchromosomal duplications on the Bactrocera oleae Y chromosome imply a distinct evolutionary origin of the sex chromosomes compared to Drosophila

BACKGROUND

Diptera have an extraordinary variety of sex determination mechanisms, and Drosophila melanogaster is the paradigm for this group. However, the Drosophila sex determination pathway is only partially conserved and the family Tephritidae affords an interesting example. The tephritid Y chromosome is postulated to be necessary to determine male development. Characterization of Y sequences, apart from elucidating the nature of the male determining factor, is also important to understand the evolutionary history of sex chromosomes within the Tephritidae. We studied the Y sequences from the olive fly, Bactrocera oleae. Its Y chromosome is minute and highly heterochromatic, and displays high heteromorphism with the X chromosome.

METHODOLOGY/PRINCIPAL FINDINGS

A combined Representational Difference Analysis (RDA) and fluorescence in-situ hybridization (FISH) approach was used to investigate the Y chromosome to derive information on its sequence content. The Y chromosome is strewn with repetitive DNA sequences, the majority of which are also interdispersed in the pericentromeric regions of the autosomes. The Y chromosome appears to have accumulated small and large repetitive interchromosomal duplications. The large interchromosomal duplications harbour an importin-4-like gene fragment. Apart from these importin-4-like sequences, the other Y repetitive sequences are not shared with the X chromosome, suggesting molecular differentiation of these two chromosomes. Moreover, as the identified Y sequences were not detected on the Y chromosomes of closely related tephritids, we can infer divergence in the repetitive nature of their sequence contents.

CONCLUSIONS/SIGNIFICANCE

The identification of Y-linked sequences may tell us much about the repetitive nature, the origin and the evolution of Y chromosomes. We hypothesize how these repetitive sequences accumulated and were maintained on the Y chromosome during its evolutionary history. Our data reinforce the idea that the sex chromosomes of the Tephritidae may have distinct evolutionary origins with respect to those of the Drosophilidae and other Dipteran families.

Darwin and the origin of interspecific genetic incompatibilities

Darwin's Origin of Species is often criticized for having little to say about speciation. The complaint focuses in particular on Darwin's supposed failure to explain the evolution of the sterility and inviability of interspecific hybrids. But in his chapter on hybridism, Darwin, working without genetics, got as close to the modern understanding of the evolution of hybrid sterility and inviability as might reasonably be expected. In particular, after surveying what was then known about interspecific crosses and the resulting hybrids, he established two facts that, while now taken for granted, were at the time radical. First, the sterility barriers between species are neither specially endowed by a creator nor directly favored by natural selection but rather evolve as incidental by-products of interspecific divergence. Second, the sterility of species hybrids results when their development is "disturbed by two organizations having been compounded into one." Bateson, Dobzhansky, and Muller later put Mendelian detail to Darwin's inference that the species-specific factors controlling development (i.e., genes) are sometimes incompatible. In this article, I highlight the major developments in our understanding of these interspecific genetic incompatibilities--from Darwin to Muller to modern theory--and review comparative, genetic, and molecular rules that characterize the evolution of hybrid sterility and inviability.

Independent fusions and recent origins of sex chromosomes in the evolution and diversification of glass knife fishes (Eigenmannia)

The genus Eigenmannia comprises several species groups that display a surprising variety of diploid chromosome numbers and sex-determining systems. In this study, hypotheses regarding phylogenetic relationships and karyotype evolution were investigated using a combination of molecular and cytogenetic methods. Phylogenetic relationships were analyzed for 11 cytotypes based on sequences from five mitochondrial DNA regions. Parsimony-based character mapping of sex chromosomes confirms previous suggestions of multiple origins of sex chromosomes. Molecular cytogenetic analyses involved chromosome painting using probes derived from whole sex chromosomes from two taxa that were hybridized to metaphases of their respective sister cytotypes. These analyses showed that a multiple XY system evolved recently (<7 mya) by fusion. Furthermore, one of the chromosomes that fused to form the neo-Y chromosome is fused independently to another chromosome in the sister cytotype. This may constitute an efficient post-mating barrier and might imply a direct function of sex chromosomes in the speciation processes in Eigenmannia. The other chromosomal sex-determination system investigated is shown to have differentiated by an accumulation of heterochromatin on the X chromosome. This has occurred in the past 0.6 my, and is the most recent chromosomal sex-determining system described to date. These results show that the evolution of sex-determining systems can proceed very rapidly.

Ninety years of Drosophila melanogaster hybrids

Within 10 years of the beginning of experimental genetic research on Drosophila melanogaster, in 1919, A. H. Sturtevant discovered its sibling species, D. simulans. He hybridized the two species and made fundamental discoveries about the genetic basis of hybrid incompatibility. The complete sterility of surviving F(1) hybrids frustrated Sturtevant and his vision of comprehensively exploring the genetics of interspecific differences. But over the next 90 years, a combination of clever genetic tricks and close observation of natural variation has led to a wealth of discovery using these and other hybrids of D. melanogaster and D. simulans, resulting in an advanced understanding of speciation and the evolution of morphology, gene regulation, and behavior.

Evolution of the genomic recombination rate in murid rodents

Although very closely related species can differ in their fine-scale patterns of recombination hotspots, variation in the average genomic recombination rate among recently diverged taxa has rarely been surveyed. We measured recombination rates in eight species that collectively represent several temporal scales of divergence within a single rodent family, Muridae. We used a cytological approach that enables in situ visualization of crossovers at meiosis to quantify recombination rates in multiple males from each rodent group. We uncovered large differences in genomic recombination rate between rodent species, which were independent of karyotypic variation. The divergence in genomic recombination rate that we document is not proportional to DNA sequence divergence, suggesting that recombination has evolved at variable rates along the murid phylogeny. Additionally, we document significant variation in genomic recombination rate both within and between subspecies of house mice. Recombination rates estimated in F(1) hybrids reveal evidence for sex-linked loci contributing to the evolution of recombination in house mice. Our results provide one of the first detailed portraits of genomic-scale recombination rate variation within a single mammalian family and demonstrate that the low recombination rates in laboratory mice and rats reflect a more general reduction in recombination rate across murid rodents.

Co-evolution of transcriptional silencing proteins and the DNA elements specifying their assembly

Co-evolution of transcriptional regulatory proteins and their sites of action has been often hypothesized but rarely demonstrated. Here we provide experimental evidence of such co-evolution in yeast silent chromatin, a finding that emerged from studies of hybrids formed between two closely related Saccharomyces species. A unidirectional silencing incompatibility between S. cerevisiae and S. bayanus led to a key discovery: asymmetrical complementation of divergent orthologs of the silent chromatin component Sir4. In S. cerevisiae/S. bayanus interspecies hybrids, ChIP-Seq analysis revealed a restriction against S. cerevisiae Sir4 associating with most S. bayanus silenced regions; in contrast, S. bayanus Sir4 associated with S. cerevisiae silenced loci to an even greater degree than did S. cerevisiae's own Sir4. Functional changes in silencer sequences paralleled changes in Sir4 sequence and a reduction in Sir1 family members in S. cerevisiae. Critically, species-specific silencing of the S. bayanus HMR locus could be reconstituted in S. cerevisiae by co-transfer of the S. bayanus Sir4 and Kos3 (the ancestral relative of Sir1) proteins. As Sir1/Kos3 and Sir4 bind conserved silencer-binding proteins, but not specific DNA sequences, these rapidly evolving proteins served to interpret differences in the two species' silencers presumably involving emergent features created by the regulatory proteins that bind sequences within silencers. The results presented here, and in particular the high resolution ChIP-Seq localization of the Sir4 protein, provided unanticipated insights into the mechanism of silent chromatin assembly in yeast.

Insights into species divergence and the evolution of hermaphroditism from fertile interspecies hybrids of Caenorhabditis nematodes

The architecture of both phenotypic variation and reproductive isolation are important problems in evolutionary genetics. The nematode genus Caenorhabditis includes both gonochoristic (male/female) and androdioecious (male/hermaprodite) species. However, the natural genetic variants distinguishing reproductive mode remain unknown, and nothing is known about the genetic basis of postzygotic isolation in the genus. Here we describe the hybrid genetics of the first Caenorhabditis species pair capable of producing fertile hybrid progeny, the gonochoristic Caenorhabditis sp. 9 and the androdioecious C. briggsae. Though many interspecies F(1) arrest during embryogenesis, a viable subset develops into fertile females and sterile males. Reciprocal parental crosses reveal asymmetry in male-specific viability, female fertility, and backcross viability. Selfing and spermatogenesis are extremely rare in XX F(1), and almost all hybrid self-progeny are inviable. Consistent with this, F(1) females do not express male-specific molecular germline markers. We also investigated three approaches to producing hybrid hermaphrodites. A dominant mutagenesis screen for self-fertile F(1) hybrids was unsuccessful. Polyploid F(1) hybrids with increased C. briggsae genomic material did show elevated rates of selfing, but selfed progeny were mostly inviable. Finally, the use of backcrosses to render the hybrid genome partial homozygous for C. briggsae alleles did not increase the incidence of selfing or spermatogenesis relative to the F(1) generation. These hybrid animals were genotyped at 23 loci, and significant segregation distortion (biased against C. briggsae) was detected at 13 loci. This, combined with an absence of productive hybrid selfing, prevents formulation of simple hypotheses about the genetic architecture of hermaphroditism. In the near future, this hybrid system will likely be fruitful for understanding the genetics of reproductive isolation in Caenorhabditis.

Chromosomes, conflict, and epigenetics: chromosomal speciation revisited

Since Darwin first noted that the process of speciation was indeed the "mystery of mysteries," scientists have tried to develop testable models for the development of reproductive incompatibilities-the first step in the formation of a new species. Early theorists proposed that chromosome rearrangements were implicated in the process of reproductive isolation; however, the chromosomal speciation model has recently been questioned. In addition, recent data from hybrid model systems indicates that simple epistatic interactions, the Dobzhansky-Muller incompatibilities, are more complex. In fact, incompatibilities are quite broad, including interactions among heterochromatin, small RNAs, and distinct, epigenetically defined genomic regions such as the centromere. In this review, we will examine both classical and current models of chromosomal speciation and describe the "evolving" theory of genetic conflict, epigenetics, and chromosomal speciation.

Progress and Promise in using Arabidopsis to Study Adaptation, Divergence, and Speciation

Fundamental questions remain to be answered on how lineages split and new species form. The Arabidopsis genus, with several increasingly well characterized species closely related to the model system A. thaliana, provides a rare opportunity to address key questions in speciation research. Arabidopsis species, and in some cases populations within a species, vary considerably in their habitat preferences, adaptations to local environments, mating system, life history strategy, genome structure and chromosome number. These differences provide numerous open doors for understanding the role these factors play in population divergence and how they may cause barriers to arise among nascent species. Molecular tools available in A. thaliana are widely applicable to its relatives, and together with modern comparative genomic approaches they will provide new and increasingly mechanistic insights into the processes underpinning lineage divergence and speciation. We will discuss recent progress in understanding the molecular basis of local adaptation, reproductive isolation and genetic incompatibility, focusing on work utilizing the Arabidopsis genus, and will highlight several areas in which additional research will provide meaningful insights into adaptation and speciation processes in this genus.

Multiple molecular mechanisms cause reproductive isolation between three yeast species

Incompatibility between nuclear and mitochondrial genomes in yeast species may represent a general mechanism of reproductive isolation during yeast evolution.

Nuclear-mitochondrial conflict (cytonuclear incompatibility) is a specific form of Dobzhansky-Muller incompatibility previously shown to cause reproductive isolation in two yeast species. Here, we identified two new incompatible genes, MRS1 and AIM22, through a systematic study of F2 hybrid sterility caused by cytonuclear incompatibility in three closely related Saccharomyces species (S. cerevisiae, S. paradoxus, and S. bayanus). Mrs1 is a nuclear gene product required for splicing specific introns in the mitochondrial COX1, and Aim22 is a ligase encoded in the nucleus that is required for mitochondrial protein lipoylation. By comparing different species, our result suggests that the functional changes in MRS1 are a result of coevolution with changes in the COX1 introns. Further molecular analyses demonstrate that three nonsynonymous mutations are responsible for the functional differences of Mrs1 between these species. Functional complementation assays to determine when these incompatible genes altered their functions show a strong correlation between the sequence-based phylogeny and the evolution of cytonuclear incompatibility. Our results suggest that nuclear-mitochondrial incompatibility may represent a general mechanism of reproductive isolation during yeast evolution.

Hybrids between species are usually inviable or sterile, possibly due to functional incompatibility between genes from the different species. Incompatible genes are hypothesized to encode interacting components that cannot function properly when paired with alleles from another species. To understand how incompatible gene pairs result in hybrid sterility or inviability, it is important to identify these genes and reconstruct their evolutionary history. A previous study has shown that incompatibility between nuclear and mitochondrial genomes (cytonuclear incompatibility) causes hybrid sterility between two yeast species. To expand on these findings, we screened three yeast species for genes involved in cytonuclear incompatibility, discovering two nuclear genes, MRS1 and AIM22, which encode proteins that are unable to support full mitochondrial function in the hybrids. Of these two genes, Mrs1 is required for removing a specific intron in the mitochondrial COX1 gene. By comparing different yeast species, we find a clear coevolutionary relationship between Mrs1 function and the COX1 intron pattern. We also show that changes in three amino acids in the Mrs1 RNA-binding domain are sufficient to make Mrs1 incompatible in hybrids. Our results suggest that cytonuclear incompatibility may represent a general mechanism of reproductive isolation during yeast evolution.

Introgression of Drosophila simulans nuclear pore protein 160 in Drosophila melanogaster alone does not cause inviability but does cause female sterility

We have been analyzing genes for reproductive isolation by replacing Drosophila melanogaster genes with homologs from Drosophila simulans by interspecific backcrossing. Among the introgressions established, we found that a segment of the left arm of chromosome 2, Int(2L)S, carried recessive genes for hybrid sterility and inviability. That nuclear pore protein 160 (Nup160) in the introgression region is involved in hybrid inviability, as suggested by others, was confirmed by the present analysis. Male hybrids carrying an X chromosome of D. melanogaster were not rescued by the Lethal hybrid rescue (Lhr) mutation when the D. simulans Nup160 allele was made homozygous or hemizygous. Furthermore, we uniquely found that Nup160 is also responsible for hybrid sterility. Females were sterile when D. simulans Nup160 was made homozygous or hemizygous in the D. melanogaster genetic background. Genetic analyses indicated that the D. simulans Nup160 introgression into D. melanogaster was sufficient to cause female sterility but that other autosomal genes of D. simulans were also necessary to cause lethality. The involvement of Nup160 in hybrid inviability and female sterility was confirmed by transgene experiment.

Hybrid incompatibility genes: remnants of a genomic battlefield?

Hybrid incompatibility (including sterility, lethality, and less extreme negative effects) interests evolutionary biologists because of its role in speciation as a reproductive isolating barrier. It also has unusual genetic properties, being mainly due to interactions between at least two genes. Recent studies have identified some of the interacting genes that underlie hybrid incompatibility. These genes represent a wide array of functions, including those involved in oxidative respiration, nuclear trafficking, DNA-binding, and plant defense. Accumulating evidence suggests genomic conflict frequently drives the divergence causing incompatibilities in hybrids. The evidence bearing on this genomic conflict hypothesis is assessed and ways to test it conclusively are suggested.

The role of meiotic drive in hybrid male sterility

Meiotic drive causes the distortion of allelic segregation away from Mendelian expected ratios, often also reducing fecundity and favouring the evolution of drive suppressors. If different species evolve distinct drive-suppressor systems, then hybrid progeny may be sterile as a result of negative interactions of these systems' components. Although the hypothesis that meiotic drive may contribute to hybrid sterility, and thus species formation, fell out of favour early in the 1990s, recent results showing an association between drive and sterility have resurrected this previously controversial idea. Here, we review the different forms of meiotic drive and their possible roles in speciation. We discuss the recent empirical evidence for a link between drive and hybrid male sterility, also suggesting a possible mechanistic explanation for this link in the context of chromatin remodelling. Finally, we revisit the population genetics of drive that allow it to contribute to speciation.

Heterochromatin, small RNA and post-fertilization dysgenesis in allopolyploid and interploid hybrids of Arabidopsis

In many plants, including Arabidopsis, hybrids between species and subspecies encounter postfertilization barriers in which hybrid seed fail to develop, or else give rise to infertile progeny. In Arabidopsis, some of these barriers are sensitive to ploidy and to the epigenetic status of donor and recipient genomes. Recently, a role has been proposed for heterochromatin in reprogramming events that occur in reproductive cells, as well as in the embryo and endosperm after fertilization. 21 nt small interfering RNA (siRNA) from activated transposable elements accumulate in pollen, and are translocated from companion vegetative cells into the sperm, while in the maturing seed 24 nt siRNA are primarily maternal in origin. Thus maternal and paternal genomes likely contribute differing small RNA to the zygote and to the endosperm. As heterochromatic sequences also differ radically between, and within, species, small RNA sequences will diverge in hybrids. If transposable elements in the seed are not targeted by small RNA from the pollen, or vice versa, this could lead to hybrid seed failure, in a mechanism reminiscent of hybrid dysgenesis in Drosophila. Heterochromatin also plays a role in apomixis and nucleolar dominance, and may utilize a similar mechanism.

Parallelism in evolution of highly repetitive DNAs in sibling species

Characterization of heterochromatin in the flour beetle Tribolium audax revealed two highly repetitive DNA families, named TAUD1 and TAUD2, which together constitute almost 60% of the whole genome. Both families originated from a common ancestral approximately 110-bp repeating unit. Tandem arrangement of these elements in TAUD1 is typical for satellite DNAs, whereas TAUD2 represents a dispersed family based on 1412-bp complex higher-order repeats composed of inversely oriented approximately 110 bp units. Comparison with repetitive DNAs in the sibling species Tribolium madens showed similarities in nucleotide sequence and length of basic repeating units and also revealed structural and organizational parallelism in tandem and dispersed families assembled from these elements. In both Tribolium species, one tandem and one dispersed family build equivalent distribution patterns in the pericentromeric heterochromatin of all chromosomes including supernumeraries. Differences in the nucleotide sequence and in the complexity of higher-order structures between families of the same type suggest a scenario according to which rearranged variants of the corresponding ancestral families were formed and distributed in genomes during or after the speciation event, following the same principles independently in each descendant species. We assume that random effects of sequence dynamics should be constrained by organizational and structural features of repeating units and possible requirements for spatial distribution of particular sequence elements. An interspersed pattern of repetitive families also points to the intensive recombination events in heterochromatin. Synergy between the meiotic bouquet stage and satellite DNA sequence dynamics could make a positive feedback loop that promotes the observed genome-wide distribution. At the same time, considering the abundance of these DNAs in heterochromatin spanning the (peri)centromeric chromosomal segments, we speculate that diverged repetitive sequences might represent the DNA basis of reproductive barrier between the two sibling species.

Altered heterochromatin binding by a hybrid sterility protein in Drosophila sibling species

Hybrid sterility of the heterogametic sex is one of the first postzygotic reproductive barriers to evolve during speciation, yet the molecular basis of hybrid sterility is poorly understood. We show that the hybrid male sterility gene Odysseus-site homeobox (OdsH) encodes a protein that localizes to evolutionarily dynamic loci within heterochromatin and leads to their decondensation. In Drosophila mauritiana x Drosophila simulans male hybrids, OdsH from D. mauritiana (OdsHmau) acts as a sterilizing factor by associating with the heterochromatic Y chromosome of D. simulans, whereas D. simulans OdsH (OdsHsim) does not. Characterization of sterile hybrid testes revealed that OdsH abundance and localization in the premeiotic phases of spermatogenesis differ between species. These results reveal that rapid heterochromatin evolution affects the onset of hybrid sterility.