Heterochrony and the evolution of poecilogony: generating larval diversity

RNA-Seq reveals divergent gene expression between larvae with contrasting trophic modes in the poecilogonous polychaete Boccardia wellingtonensis

The polychaete Boccardia wellingtonensis is a poecilogonous species that produces different larval types. Females may lay Type I capsules, in which only planktotrophic larvae are present, or Type III capsules that contain planktotrophic and adelphophagic larvae as well as nurse eggs. While planktotrophic larvae do not feed during encapsulation, adelphophagic larvae develop by feeding on nurse eggs and on other larvae inside the capsules and hatch at the juvenile stage. Previous works have not found differences in the morphology between the two larval types; thus, the factors explaining contrasting feeding abilities in larvae of this species are still unknown. In this paper, we use a transcriptomic approach to study the cellular and genetic mechanisms underlying the different larval trophic modes of B. wellingtonensis. By using approximately 624 million high-quality reads, we assemble the de novo transcriptome with 133,314 contigs, coding 32,390 putative proteins. We identify 5221 genes that are up-regulated in larval stages compared to their expression in adult individuals. The genetic expression profile differed between larval trophic modes, with genes involved in lipid metabolism and chaetogenesis over expressed in planktotrophic larvae. In contrast, up-regulated genes in adelphophagic larvae were associated with DNA replication and mRNA synthesis.

Baby makes three: Maternal, paternal, and zygotic genetic effects shape larval phenotypic evolution

The evolutionary potential of a population is shaped by the genetic architecture of its life-history traits. Early-life phenotypes are influenced by both maternal and offspring genotype, and efforts to understand life-history evolution therefore require consideration of the interactions between these separate but correlated genomes. We used a four-generation experimental pedigree to estimate the genetic architecture of early-life phenotypes in a species with dramatic variation in larval size and morphology. In the polychaete annelid Streblospio benedicti, females make either many small eggs that develop into complex larvae that feed in the plankton or few large eggs that develop into benthic juveniles without having to feed as larvae. By isolating the contributions of maternal, paternal, and zygotic genotype to larval traits, we determined that larval anatomical structures are governed by the offspring genotype at a small number of large-effect loci. Larval size is not shaped by the larva's own genotype but instead depends on loci that act in the mother, and at two genomic locations, by loci that act in the father. The overall phenotype of each larva thus depends on three separate genomes, and a population's response to selection on larval traits will reflect the interactions among them.

Brood Reduction, Not Poecilogony, in a Vermetid Gastropod with Two Developmental Outcomes within Egg Capsules

A small vermetid gastropod broods capsules containing nurse eggs and embryos that develop into small veligers. A few of these veligers continue development and growth while nurse eggs and developmentally arrested sibling veligers disappear. Survivors hatch as crawling pediveligers and juveniles. None of the veligers, if removed from capsules, swim in a directed way or withdraw into their shells, indicating that even the developing veligers are unsuited for extracapsular life until they can crawl. The shells of arrested veligers decalcify while their siblings grow. Few of the developmentally arrested veligers that were isolated from siblings and fed algal cells resumed detectable growth. Nurse eggs rather than cannibalism provide most of the food, but full growth of developing veligers depends on limited sharing; arrest of some siblings is a necessary adjunct of the nurse-egg feeding. Here, two developmental outcomes for larvae produced by developmental arrest of some (often termed poecilogony) serves instead as a means of brood reduction. Brood reduction is often attributed to family conflicts resulting from genetic differences. Another hypothesis is that a mother who cannot accurately sort numbers of nurse eggs and developing eggs into capsules could rely on brood reduction to adjust food for her offspring. At the extreme, an entirely random packaging would produce a binomial distribution of embryos in capsules, a very uneven distribution of food per embryo, and some capsules with no embryos. Males have yet to be found in this species, but even if reproduction is asexual, selection could still favor brood reduction.

Heterokairy: a significant form of developmental plasticity?

There is a current surge of research interest in the potential role of developmental plasticity in adaptation and evolution. Here we make a case that some of this research effort should explore the adaptive significance of heterokairy, a specific type of plasticity that describes environmentally driven, altered timing of development within a species. This emphasis seems warranted given the pervasive occurrence of heterochrony, altered developmental timing between species, in evolution. We briefly review studies investigating heterochrony within an adaptive context across animal taxa, including examples that explore links between heterokairy and heterochrony. We then outline how sequence heterokairy could be included within the research agenda for developmental plasticity. We suggest that the study of heterokairy may be particularly pertinent in (i) determining the importance of non-adaptive plasticity, and (ii) embedding concepts from comparative embryology such as developmental modularity and disassociation within a developmental plasticity framework.

Differences in the timing of cardio-respiratory development determine whether marine gastropod embryos survive or die in hypoxia

Physiological plasticity of early developmental stages is a key way by which organisms can survive and adapt to environmental change. We investigated developmental plasticity of aspects of the cardio-respiratory physiology of encapsulated embryos of a marine, gastropod Littorina obtusata surviving exposure to moderate hypoxia (pO2=8 kPa) and compared the development of these survivors with that of individuals that died before hatching. Individuals surviving hypoxia exhibited a slower rate of development and altered ontogeny of cardio-respiratory structure and function compared with normoxic controls (pO2>20 kPa). The onset and development of the larval and adult hearts were delayed in chronological time in hypoxia, but both organs appeared earlier in developmental time and cardiac activity rates were greater. The velum, a transient, ‘larval’ organ thought to play a role in gas exchange, was larger in hypoxia but developed more slowly (in chronological time), and velar cilia-driven, rotational activity was lower. Despite these effects of hypoxia, 38% of individuals survived to hatching. Compared with those embryos that died during development, these surviving embryos had advanced expression of adult structures, i.e. a significantly earlier occurrence and greater activity of their adult heart and larger shells. In contrast, embryos that died retained larval cardio-respiratory features (the velum and larval heart) for longer in chronological time. Surviving embryos came from eggs with significantly higher albumen provisioning than those that died, suggesting an energetic component for advanced development of adult traits.

A new case of poecilogony from South America and the implications of nurse eggs, capsule structure, and maternal brooding behavior on the development of different larval types

Poecilogony is the production of different larval types within the same species. Although rare, poecilogonous species are ideal systems for testing the evolutionary and ecological implication of different developmental modes in marine invertebrates. Here, we described a new case of poecilogony, the Southern Hemisphere spionid Boccardia wellingtonensis. We used a combination of common-garden experiments, video recordings, and in vitro manipulations of individuals from three sites to (1) document the type of poecilogony, the brooding behavior of the mother, and the hatching process; (2) experimentally measure the effect of nurse eggs on the growth and type of larvae produced; and (3) document variation in the length of the brooding period, number of capsules, larvae, and nurse eggs of mothers from three sites to explore the potential for plasticity in reproductive traits. These results were compared to the previously reported poecilogonous species B. proboscidea, which resembles B. wellingtonensis in size, morphology, ecology, and reproductive strategy but differs in capsule structure. We found that in contrast to B. proboscidea, B. wellingtonensis produced larvae that, in isolation and in the presence of nurse eggs, developed into a wide range of offspring sizes. Mothers brood and hatch the larvae with frequent partial hatching of the brood during the brooding period. Although larvae could not liberate themselves, larvae crossed to other capsules as interconnections between capsules broke during the developmental period, potentially affecting food availability, sibling competition for nurse eggs, and cannibalism. Variation in brooding time and number of capsules deposited among sites suggest local adaptations.

Effects of dispersal plasticity on population divergence and speciation

Phenotypic plasticity is thought to have a role in driving population establishment, local adaptation and speciation. However, dispersal plasticity has been underappreciated in this literature. Plasticity in the decision to disperse is taxonomically widespread and I provide examples for insects, molluscs, polychaetes, vertebrates and flowering plants. Theoretical work is limited but indicates an interaction between dispersal distance and plasticity in the decision to disperse. When dispersal is confined to adjacent patches, dispersal plasticity may enhance local adaptation over unconditional (non-plastic) dispersal. However, when dispersal distances are greater, plasticity in dispersal decisions strongly reduces the potential for local adaptation and population divergence. Upon dispersal, settlement may be random, biased but genetically determined, or biased but plastically determined. Theory shows that biased settlement of either type increases population divergence over random settlement. One model suggests that plasticity further enhances chances of speciation. However, there are many strategies for deciding on where to settle such as a best-of-N strategy, sequential sampling with a threshold for acceptance or matching with natal habitat. To date, these strategies do not seem to have been compared within a single model. Although we are just beginning to explore evolutionary effects of dispersal plasticity, it clearly has the potential to enhance as well as inhibit population divergence. Additional work should pay particular attention to dispersal distance and the strategy used to decide on where to settle.

Hox gene expression during postlarval development of the polychaete Alitta virens

BACKGROUND

Hox genes are the family of transcription factors that play a key role in the patterning of the anterior-posterior axis of all bilaterian animals. These genes display clustered organization and colinear expression. Expression boundaries of individual Hox genes usually correspond with morphological boundaries of the body. Previously, we studied Hox gene expression during larval development of the polychaete Alitta virens (formerly Nereis virens) and discovered that Hox genes are expressed in nereid larva according to the spatial colinearity principle. Adult Alitta virens consist of multiple morphologically similar segments, which are formed sequentially in the growth zone. Since the worm grows for most of its life, postlarval segments constantly change their position along the anterior-posterior axis.

RESULTS

We studied the expression dynamics of the Hox cluster during postlarval development of the nereid Alitta virens and found that 8 out of 11 Hox genes are transcribed as wide gene-specific gradients in the ventral nerve cord, ectoderm, and mesoderm. The expression domains constantly shift in accordance with the changing proportions of the growing worm, so expression domains of most Hox genes do not have stable anterior or/and posterior boundaries.In the course of our study, we revealed long antisense RNA (asRNA) for some Hox genes. Expression patterns of two of these genes were analyzed using whole-mount in-situ hybridization. This is the first discovery of antisense RNA for Hox genes in Lophotrochozoa.

CONCLUSION

Hox gene expression in juvenile A. virens differs significantly from Hox gene expression patterns both in A. virens larva and in other Bilateria.We suppose that the postlarval function of the Hox genes in this polychaete is to establish and maintain positional coordinates in a constantly growing body, as opposed to creating morphological difference between segments.

Phylogeography and reproductive variation of the poecilogonous polychaete Boccardia proboscidea (Annelida: Spionidae) along the West Coast of North America

The ability to produce more than one kind of offspring, or poecilogony, is a striking example of reproductive variability. Traditionally, larval nutrition has been classified as a dichotomy: if offspring obtain nutrition from their mothers (lecithotrophy), there is lower fecundity and greater chance of offspring survival than when they get their nutrition from plankton (planktotrophy). The polychaete Boccardia proboscidea (Spionidae) produces both types of embryos using three different reproductive strategies. In this study, we examined the roles of genetic history and phenotypic plasticity on explaining natural variation in B. proboscidea along the Pacific coast of the United States using two genetic mitochondrial markers, 16S rDNA and Cyt b, and common garden experiments. These data show a single North American West Coast network that is structured, geographically, by the well-documented biogeographic break near Point Conception, California. The southern group within this network covers a smaller range, but has larger haplotype diversity, than the northern group. Some individuals differing in reproductive type had the same haplotype, indicating independence of these features; however, differences between laboratory and field data suggest additional geographic variation within one of the reproductive types. Females from higher latitudes provide offspring with larger supplies of extra embryonic nutrition than females from southern latitudes. Results herein suggest that both genetic history and developmental plasticity are playing a role in the maintenance of this reproductive polymorphism.

Holding on to a shifting substrate: plasticity of egg mass tethers and tethering forces in soft sediment for an intertidal gastropod

Staying attached at a favorable site can be a major challenge for organisms in flow. Meeting this challenge depends on properties of the attachment structure and substrate, the nature of fluid flow, and the ability to adjust attachment force in response to hydrodynamic conditions. A broad taxonomic range of adult stages use adhesion or suction to attach to hard substrates in intertidal habitats, which experience flow from waves and tidal currents. We address the unique challenges of attachment to soft sediment in reproductive structures deposited on tidal flats. Egg masses of the opisthobranch mollusc Melanochlamys diomedea are anchored to the sediment by a buried tether composed of gel and sediment. In the field, populations differed in absolute tethering force and tethering force per unit size (= tenacity). Population differences in tenacity persisted for egg masses oviposited under common conditions in the laboratory. Adults exposed to greater flow produced tethers with greater tenacity but without an increase in tether size. Tethers tended to fail by slippage rather than breakage, indicating that tethering force depends more on frictional interaction with sediment than on strength of the tether axis. These results suggest that adults respond to variation in risks of embryo dislodgment by adjusting the tethering properties of egg masses, and that these adjustments involve more than simple changes in tether length or mass.

Phylogenetic analysis of cryptic speciation in the polychaete Pygospio elegans

Development in marine invertebrate species can take place through a variety of modes and larval forms, but within a species, developmental mode is typically uniform. Poecilogony refers to the presence of more than one mode of development within a single species. True poecilogony is rare, however, and in some cases, apparent poecilogony is actually the result of variation in development mode among recently diverged cryptic species. We used a phylogenetic approach to examine whether poecilogony in the marine polychaete worm, Pygospio elegans, is the result of cryptic speciation. Populations of worms identified as P. elegansooded, and intermediate larvae; these modes are found both within and among populations. We examined sequence variation among partial mitochondrial cytochrome c oxidase subunit I sequences obtained for 279 individual worms sampled across broad geographic and environmental scales. Despite a large number of unique haplotypes (121 haplotypes from 279 individuals), sequence divergence among European samples was low (1.7%) with most of the sequence variation observed within populations, relative to the variation among regions. More importantly, we observed common haplotypes that were widespread among the populations we sampled, and the two most common haplotypes were shared between populations differing in developmental mode. Thus, our results support an earlier conclusion of poecilogony in P elegans. In addition, predominantly planktonic populations had a larger number of population-specific low-frequency haplotypes. This finding is largely consistent with interspecies comparisons showing high diversity for species with planktonic developmental modes in contrast to low diversity in species with brooded developmental modes.

Effects of temperature on hatching time and hatchling proportions in a poecilogonous population of Haminoea zelandiae

Poecilogony is a relatively uncommon life-history strategy that results in the production of two different larval forms from the same egg mass (e.g., free-swimming lecithotrophic larvae and post-metamorphic, crawling juveniles). In this study, a population of the opisthobranch gastropod Haminoea zelandiae from Pauatahanui Inlet, New Zealand, was found to exhibit poecilogony. Further, differences in development, hatching times and proportion of hatchlings that were veligers or juveniles were examined for egg masses in two temperature regimes in the laboratory: cool (15-17 °C), and warm (21-23 °C). Hatching proportions were also examined for egg masses collected from the field (where temperatures ranged from 21-23 °C) for varying lengths of time (1 d, 5 d, and 10 d post-spawning). Hatchlings from egg masses in warmer temperatures developed faster and hatched earlier than those in cool temperatures. In the laboratory, egg masses in warm conditions hatched a greater proportion of post-metamorphic juveniles (45.4%) compared to egg masses in cool conditions (24.6%) Further, egg masses that had been in the field 10 d before hatching (i.e., more days at warmer temperatures) exhibited a greater proportion of post-metamorphic juveniles (67.9%) than those that were collected after only 1 d in the field (25.1%). Together these results suggest that temperature may have an important role in mediating dispersal strategies in this poecilogonous species.

Physics and the canalization of morphogenesis: a grand challenge in organismal biology

Morphogenesis takes place against a background of organism-to-organism and environmental variation. Therefore, fundamental questions in the study of morphogenesis include: How are the mechanical processes of tissue movement and deformation affected by that variability, and in turn, how do the mechanic of the system modulate phenotypic variation? We highlight a few key factors, including environmental temperature, embryo size and environmental chemistry that might perturb the mechanics of morphogenesis in natural populations. Then we discuss several ways in which mechanics-including feedback from mechanical cues-might influence intra-specific variation in morphogenesis. To understand morphogenesis it will be necessary to consider whole-organism, environment and evolutionary scales because these larger scales present the challenges that developmental mechanisms have evolved to cope with. Studying the variation organisms express and the variation organisms experience will aid in deciphering the causes of birth defects.