Polymorphism in developmental mode and its effect on population genetic structure of a spionid polychaete, Pygospio elegans

Population genetic structure of sedentary marine species is expected to be shaped mainly by the dispersal ability of their larvae. Long-lived planktonic larvae can connect populations through migration and gene flow, whereas species with nondispersive benthic or direct-developing larvae are expected to have genetically differentiated populations. Poecilogonous species producing different larval types are ideal when studying the effect of developmental mode on population genetic structure and connectivity. In the spionid polychaete Pygospio elegans, different larval types have been observed between, and sometimes also within, populations. We used microsatellite markers to study population structure of European P. elegans from the Baltic Sea (BS) and North Sea (NS). We found that populations with planktonic larvae had higher genetic diversity than did populations with benthic larvae. However, this pattern may not be related to developmental mode, since in P. elegans, developmental mode may be associated with geography. Benthic larvae were more commonly seen in the brackish BS and planktonic larvae were predominant in the NS, although both larval types also are found from both areas. Significant isolation-by-distance (IBD) was found overall and within regions. Most of the pair-wise F(ST) comparisons among populations were significant, although some geographically close populations with planktonic larvae were found to be genetically similar. However, these results, together with the pattern of IBD, autocorrelation within populations, as well as high estimated local recruitment, suggest that dispersal is limited in populations with planktonic larvae as well as in those with benthic larvae. The decrease in salinity between the NS and BS causes a barrier to gene flow in many marine species. In P. elegans, low, but significant, differentiation was detected between the NS and BS (3.34% in AMOVA), but no clear transition zone was observed, indicating that larvae are not hampered by the change in salinity.     

Poecilogony and population genetic structure in Elysia pusilla (Heterobranchia: Sacoglossa), and reproductive data for five sacoglossans that express dimorphisms in larval development

Credible cases of poecilogony, the production of two distinct larval morphs within a species, are extremely rare in marine invertebrates, yet peculiarly common in a clade of herbivorous sea slugs, the Sacoglossa. Only five animal species have been reported to express dimorphic egg sizes that result in planktotrophic and lecithotrophic larvae: the spionid polychaete Streblospio benedicti and four sacoglossans distributed in temperate estuaries or the Caribbean. Here, we present developmental and genetic evidence for a fifth case of poecilogony via egg-size dimorphism in the Sacoglossa and the first example from the tropical Indo-Pacific. The sea slug Elysia pusilla produced both planktotrophic and lecithotrophic larvae in Guam and Japan. Levels of genetic divergence within populations were markedly low and rule out cryptic species. However, divergence among populations was exceptionally high (10-12% at the mitochondrial cytochrome c oxidase I locus), illustrating that extensive phylogeographic structure can persist in spite of the dispersal potential of planktotrophic larvae. We review reproductive, developmental, and ecological data for the five known cases of poecilogony in the Sacoglossa, including new data for Costasiella ocellifera from the Caribbean. We hypothesize that sacoglossans achieve lecithotrophy at smaller egg sizes than do related clades of marine heterobranchs, which may facilitate developmental plasticity that is otherwise vanishingly rare among animals. Insight into the environmental drivers and evolutionary results of shifts in larval type will continue to be gleaned from population-level studies of poecilogonous taxa like E. pusilla, and should inform life-history theory about the causes and consequences of alternative development modes in marine animals.     

Morphological and genetic variation indicate cryptic species within Lamarck's little sea star, Parvulastra (=Patiriella) exigua

The asterinid sea star Parvulastra exigua (Lamarck) is a common member of temperate intertidal marine communities from geographically widespread sites around the southern hemisphere. Individuals from Australian populations lay benthic egg masses (through orally directed gonopores) from which nonplanktonic offspring hatch and metamorphose without a dispersing planktonic larval phase. Scattered reports in the taxonomic literature refer to a similar form in southern Africa with aborally directed gonopores (and possibly broadcast spawning of planktonic eggs and larvae); such differences would be consistent with cryptic species variation. Surveys of morphology and mtDNA sequences have revealed cryptic species diversity in other asterinid genera. Here we summarize the taxonomic history of Lamarck's "Astérie exigu?" and survey morphological variation (the location of the gonopores) for evidence that some P. exigua populations include cryptic species with a different mode of reproduction. We found strong evidence for multiple species in the form of two phenotypes and modes of reproduction (oral and aboral gonopore locations) in populations from southern Africa and islands in the Atlantic and Indian oceans. Both modes of reproduction have broad geographic ranges. These results are consistent with previously published genetic data that indicate multiple species in African and island (but not Australian) populations.     

Extremely high molecular diversity within the East Asian nematode Caenorhabditis sp. 5

Most relatives of the self-fertilizing hermaphroditic nematode model organism Caenorhabditis elegans reproduce via obligate outbreeding between males and females, which also represents the ancestral mode of reproduction within the genus. However, little is known about the scope of genetic diversity and differentiation within such gonochoristic species, especially those found outside of temperate Europe and North America. It is critical to understand the evolutionary processes operating in these species to provide a framework for deciphering the evolution of hermaphroditism and a baseline for the application of outcrossing Caenorhabditis to problems in evolutionary genetics. Here, we investigate for the first time molecular sequence variation for Caenorhabditis sp. 5, a species found commonly in eastern Asia. We identify enormous levels of standing genetic variation that approach the levels observed in the marine broadcast-spawning sea squirt, Ciona savignyi. Although we document significant isolation by distance, we demonstrate that the high polymorphism within C. sp. 5 is not because of strong differentiation among populations or to the presence of cryptic species. These findings illustrate that molecular population genetic approaches to studying obligately outbreeding species of Caenorhabditis will prove powerful in identifying and characterizing functionally and evolutionarily important features of the genome.     

Evolution of poecilogony from planktotrophy: cryptic speciation, phylogeography, and larval development in the gastropod genus Alderia

Poecilogony, a rare phenomenon in marine invertebrates, occurs when alternative larval morphs differing in dispersal potential or trophic mode are produced from a single genome. Because both poecilogony and cryptic species are prevalent among sea slugs in the suborder Sacoglossa (Gastropoda: Opisthobranchia), molecular data are needed to confirm cases of variable development and to place them in a phylogenetic context. The nominal species Alderia modesta produces long-lived, feeding larvae throughout the North Atlantic and Pacific, but in California can also produce short-lived larvae that metamorphose without feeding. We collected morphological, developmental, and molecular data for Alderia from 17 sites spanning the eastern and western Pacific and North Atlantic. Estuaries south of Bodega Harbor, California, contained a cryptic species (hereafter Alderia sp.) with variable development, sister to the strictly planktotrophic A. modesta. The smaller Alderia sp. seasonally toggled between planktotrophy and lecithotrophy, with some individuals differing in development but sharing mitochondrial DNA haplotypes. The sibling species overlapped in Tomales Bay, California, but showed no evidence of hybridization; laboratory mating trials suggest postzygotic isolation has arisen. Intra- and interspecific divergence times were estimated using a molecular clock calibrated with geminate sacoglossans. Speciation occurred about 4.1 million years ago during a major marine radiation in the eastern Pacific, when large inland embayments in California may have isolated ancestral populations. Atlantic and Pacific A. modesta diverged about 1.7 million years ago, suggesting trans-Arctic gene flow was interrupted by Pleistocene glaciation. Both Alderia species showed evidence of late Pleistocene population expansion, but the southern Alderia sp. likely experienced a more pronounced bottleneck. Reduced body size may have incurred selection against obligate planktotrophy in Alderia sp. by limiting fecundity in the face of high larval mortality rates in warm months. Alternatively, poecilogony may be an adaptive response to seasonal opening of estuaries, facilitating dispersal by long-lived larvae. An improved understanding of the forces controlling seasonal shifts in development in Alderia sp. may yield insight into the evolutionary forces promoting transitions to nonfeeding larvae.     

Introduction to symposium: Poecilogony--a window on larval evolutionary transitions in marine invertebrates

Poecilogony is the intraspecific variation in developmental mode that has been described in some marine invertebrates. Poecilogonous species produce different larval forms (e.g., free-swimming planktotrophic larvae as well as brooded lecithotrophic or adelphophagic larvae). Poecilogony can be a controversial topic, since it is difficult to identify and characterize the phenomenon with certainty. It has been challenging to determine whether poecilogony represents developmental polymorphism with a genetic basis or developmental polyphenism reflecting plastic responses to environmental cues. Other outstanding questions include whether common mechanisms underlie the developmental variation we observe in poecilogonous species, and whether poecilogony is maintained in different taxa through similar mechanisms or selective pressures. Poecilogonous species provide a unique opportunity to elucidate the cellular, developmental, and genetic mechanisms underlying evolutionary transitions in developmental mode, as well as to help clarify the selective pressures and possible ecological circumstances that might be involved. Here, we describe an integrative approach to the study of poecilogony and its role in larval evolutionary transitions highlighted during a symposium held at the 2012 annual meeting of the Society for Integrative and Comparative Biology.     

The effects of mode of development on phylogeography and population structure of North Atlantic Crepidula (Gastropoda: Calyptraeidae)

The mode of development of marine invertebrates is thought to influence levels of population structure and the location of species range endpoints via differences in dispersal ability. To examine these effects, populations of three sympatric clades of sedentary, marine gastropods in the genus Crepidula were sampled along the Atlantic and Gulf coasts of North America. A haplotype tree was constructed for each clade based on 640 bp sequences of mitochondrial cytochrome oxidase c subunit I. Examination of the tree topology, and AMOVA analysis show that species with direct development (those hatching as benthic juveniles) have higher levels of population structure than do species with planktonic development. Both species in the direct-developing C. convexa clade have high levels of geographical differentiation, with most populations representing a discrete clade of haplotypes. The planktotrophic species C. fornicata contains two major haplotype clades, both of which include samples from throughout the Atlantic coast. In this species there is no geographical differentiation among haplotypes but AMOVA analysis detects a small but statistically significant level of geographical structure. The population structure within the C. plana species complex appears also to vary with mode of development: C. atrasolea, a direct-developing species, has higher levels of population structure than does C. depressa, a sympatric planktotrophic species. The coincident occurrence of range endpoints and genetic breaks along the east coast of Florida in both direct-developing species and species with planktonic development indicates that this biogeographic break is not due to development-specific mechanisms such as hydrographic effects on larval recruitment.     

High local and global diversity of Flavobacteria in marine plankton

Members of the phylum Bacteroidetes are among the most abundant microbes in coastal marine waters, but it is unclear to which extent the diversity within this phylum is covered by currently available 16S rRNA gene sequence information. We, thus, obtained a comprehensive collection of sequence types affiliated with Bacteroidetes in coastal North Sea surface waters and we compared this local diversity with the available sequences of marine planktonic and other aquatic Bacteroidetes. Approximately 15% of > 600 clones from two libraries (August 2000, June 2001) were related to Bacteroidetes, specifically to the Flavobacteria. Local diversity appeared to be almost exhaustively sampled. However, the diversity of the two libraries virtually did not overlap, indicating a pronounced temporal variability of the planktonic Flavobacteria assemblage. The majority of sequence types represented novel phylogenetic lineages, adding 6-7% to the currently known genera and species of Bacteroidetes in marine waters. Different diversity estimators suggested that so far only approximately half of the global diversity of planktonic marine Bacteroidetes has been described. The data set moreover indicated that cultivation-independent techniques and isolation approaches have recovered almost equally sized and virtually non-overlapping fractions of the currently known diversity within this phylum. Interestingly, only 15% of genera of Bacteroidetes from various aquatic environments appear to occur in more than one habitat type.     

Evidence for subdivision of the root-endophyte Phialocephala fortinii into cryptic species and recombination within species

The genetic structure of the root-endophyte Phialocephala fortinii was analyzed in three study sites using 11 single-copy RFLP probes. A total of 541 strains isolated from surface-sterilized, fine roots (diameter 0.5-3 mm) of Norway spruce (Picea abies) were examined. The average gene diversity (H) was high in all three study sites. Cluster analysis showed that up to four well-separated clusters of multi-locus haplotypes were present within the sites. Significant population subdivision was detected among these clusters, suggesting that groups of multi-locus haplotypes were reproductively isolated and that P. fortinii is a species complex composed of several cryptic species. This hypothesis was supported by ISSR-PCR which showed clusters consistent with those of the multi-locus haplotypes identified by RFLP analysis. In contrast, ITS sequence analysis did not allow to separate the species as clearly. The index of association (IA) did not deviate significantly from zero within any cryptic species, suggesting that recombination occurs within these species. Cryptic species occurred sympatrically. Thalli of two cryptic species were detected in the same 5-mm-long root segment in one instance. No significant differentiation was observed among populations of the same cryptic species in forest stands located approximately 5 km from each other. This finding is consistent with significant gene flow over this spatial scale. In addition, several isolates with both identical multi-locus haplotype and identical ISSR fingerprint were found at each study site indicating genotype flow or a recent common history between study sites.     

Phylogeography of Peromyscus furvus (Rodentia; muridae) based on cytochrome b sequence data

We conducted phylogenetic analyses of cytochrome b sequence data to assess genetic variation within and among the three allopatric segments of the distribution of Peromyscus furvus from the Sierra Madre Oriental in eastern Mexico. We identified 24 unique haplotypes among the 54 individuals examined and genetic distances ranged up to 0.078 substitutions per site. Populations from the central portion of the range formed a monophyletic unit, whereas samples from the southern distributional unit were polyphyletic. Furthermore, the southernmost population sampled may represent a distinct species. This high degree of genetic differentiation among populations, currently recognized as conspecific, mirrors the result of other genetic studies of highland rodents in Mesoamerica. Together these studies indicate that the region, already considered hyperdiverse on the basis of species diversity and endemism, may contain considerably greater diversity than is currently appreciated.     

Patterns of nucleotide polymorphism distinguish temperate and tropical wild isolates of Caenorhabditis briggsae

Caenorhabditis briggsae provides a natural comparison species for the model nematode C. elegans, given their similar morphology, life history, and hermaphroditic mode of reproduction. Despite C. briggsae boasting a published genome sequence and establishing Caenorhabditis as a model genus for genetics and development, little is known about genetic variation across the geographic range of this species. In this study, we greatly expand the collection of natural isolates and characterize patterns of nucleotide variation for six loci in 63 strains from three continents. The pattern of polymorphisms reveals differentiation between C. briggsae strains found in temperate localities in the northern hemisphere from those sampled near the Tropic of Cancer, with diversity within the tropical region comparable to what is found for C. elegans in Europe. As in C. elegans, linkage disequilibrium is pervasive, although recombination is evident among some variant sites, indicating that outcrossing has occurred at a low rate in the history of the sample. In contrast to C. elegans, temperate regions harbor extremely little variation, perhaps reflecting colonization and recent expansion of C. briggsae into northern latitudes. We discuss these findings in relation to their implications for selection, demographic history, and the persistence of self-fertilization.     

Robustness and flexibility in nematode vulva development

The Caenorhabditis elegans vulva has served as a paradigm for how conserved developmental pathways, such as EGF-Ras-MAPK, Notch and Wnt signaling, participate in networks driving animal organogenesis. Here, we discuss an emerging direction in the field, which places vulva research in a quantitative and microevolutionary framework. The final vulval cell fate pattern is known to be robust to change, but only recently has the variation of vulval traits been measured under stochastic, environmental or genetic variation. Whereas the resulting cell fate pattern is invariant among rhabditid nematodes, recent studies indicate that the developmental system has accumulated cryptic variation, even among wild C. elegans isolates. Quantitative differences in the signaling network have emerged through experiments and modeling as the driving force behind cryptic variation in Caenorhabditis species. On a wider evolutionary scale, the establishment of new model species has informed about the presence of qualitative variation in vulval signaling pathways.     

Hidden diversity and host specificity in cycliophorans: a phylogeographic analysis along the North Atlantic and Mediterranean Sea

In order to elucidate the evolutionary history and the population structure of the members of the phylum Cycliophora, which live commensally on three species of lobsters, we studied sequence variation in the mitochondrial gene cyctochrome c oxidase subunit I. Overall 242 sequences from 16 locations on both coasts of the North Atlantic, including the North Sea and the Mediterranean, were analysed, revealing 28 haplotypes, with a maximum sequence divergence of 16.6%. Total genetic diversity was high (h = 0.8322, pi = 0.0898), as it was for the commensals on Homarus americanus (17 haplotypes, h = 0.7506, pi = 0.0504). However, it was low for commensals on Nephrops norvegicus (6 haplotypes, h = 0.3899, pi = 0.0035), and intermediate for cycliophorans on Homarus gammarus (5 haplotypes, h = 0.3020, pi = 0.0140). Although two of the host lobsters co-inhabit the coastal waters of Europe, a strong genetic structure (78.45% of the observed genetic variation) was detected among populations on all host species, indicating the existence of a reproductively isolated species on each lobster. In addition, genetic structure over long distances exists among populations on each host species. Such patterns can be explained by the limited dispersal ability of the cycliophoran chordoid larva. Demographic and phylogenetic analyses suggest old and possibly cryptic populations present on H. americanus and H. gammarus, while the latter may have experienced recent bottlenecks, perhaps during Pleistocene glaciations. Populations on N. norvegicus appear to be of recent origin.     

Molecular phylogenetic perspective on evolution of lizards of the Anolis grahami series

We report the results of phylogenetic analyses of 1447 bases of mitochondrial DNA sequence for 21 populations representing seven species of the Anolis grahami series (A. conspersus, A. garmani, A. grahami, A. lineatopus, A. opalinus, A. reconditus, and A. valencienni), six of which occur on Jamaica. These data include 705 characters that are phylogenetically informative according to parsimony. A parsimony analysis of these data combined with previously published allozymic data yields a single most parsimonious tree with strong support for monophyly of the A. grahami series, the sister-group relationship between Anolis lineatopus and A. reconditus and a clade composed of Anolis garmani, A. grahami, and A. opalinus. Based on DNA data alone, A. conspersus is nested within A. grahami. Haplotypes sampled from geographic populations of A. grahami, A. lineatopus, and A. opalinus are highly divergent (approximately 12-15% sequence difference on average for each species) and show similar phylogeographic patterns, suggesting that each of these currently recognized species may be a complex of species. Anolis valencienni also shows high sequence divergence among haplotypes from different geographic populations (approximately 8% sequence difference) and may contain cryptic species. Divergence among haplotypes within A. garmani is substantially lower (approximately 3% sequence difference), and phylogeographic patterns are significantly different from those observed in A. grahami, A. lineatopus and A. opalinus.     

Vertical niche partitioning between cryptic sibling species of a cosmopolitan marine planktonic protist

A large portion of the surface‐ocean biomass is represented by microscopic unicellular plankton. These organisms are functionally and morphologically diverse, but it remains unclear how their diversity is generated. Species of marine microplankton are widely distributed because of passive transport and lack of barriers in the ocean. How does speciation occur in a system with a seemingly unlimited dispersal potential? Recent studies using planktonic foraminifera as a model showed that even among the cryptic genetic diversity within morphological species, many genetic types are cosmopolitan, lending limited support for speciation by geographical isolation. Here we show that the current two‐dimensional view on the biogeography and potential speciation mechanisms in the microplankton may be misleading. By depth‐stratified sampling, we present evidence that sibling genetic types in a cosmopolitan species of marine microplankton, the planktonic foraminifer Hastigerina pelagica, are consistently separated by depth throughout their global range. Such strong separation between genetically closely related and morphologically inseparable genetic types indicates that niche partitioning in marine heterotrophic microplankton can be maintained in the vertical dimension on a global scale. These observations indicate that speciation along depth (depth‐parapatric speciation) can occur in vertically structured microplankton populations, facilitating diversification without the need for spatial isolation.     

Two oceans, two taxa and one mode of development: latitudinal diversity patterns of South American crabs and test for possible causal processes

The latitudinal gradient of species diversity is a widely recognized but poorly understood phenomenon. In marine systems, differences in dispersal abilities among species may pose an additional problem in identifying the processes that affect diversity. We compared latitudinal diversity gradients along two parallel continental coasts, the east and west coasts of South America, of two groups of Crustacea (Brachyura and Anomura), which exclusively exhibit planktonic development. We also evaluated the species‐area and the energy‐input hypotheses. Diversity decreased with increasing latitude for both groups in both oceans. Results suggest that the spatial structure of sea surface temperature (SST) explains diversity of both groups at large, but not small (< 5°), scales. Range size and latitude were not correlated. We hypothesize that SST differentially affects taxa with contrasting modes of development, influencing patterns of diversity. We suggest that developmental modes of marine organisms should be considered in future diversity analyses.     

Heterochrony and the evolution of poecilogony: generating larval diversity

Poecilogony is the production of more than one type of young within a single species of marine invertebrate. We chose a poecilogonous polychaete to investigate potential differences in morphogenesis among offspring that are polymorphic in dispersal potentials (planktonic, benthic) and trophic modes (planktotrophy, adelphophagy). Differences in morphogenesis occur and are strongly influenced by maternal type. Females that provide extra-embryonic nutrition (as nurse eggs; type III females) also produce offspring with an accelerated onset of juvenile traits, relative to planktotrophic offspring of females that do not provide extra-embryonic nutrition (type I females). Thus, progeny of some females appear morphologically preadapted for a benthic lifestyle. Surprisingly, differences in phenotype among offspring do not parallel offspring ecotype, as offspring with early onset of juvenile traits (III) are ecologically bimodal. Some Type III offspring eat the nurse eggs (adelphophagy), have accelerated development, and hatch as benthic juveniles. In contrast, their siblings hatch as small, planktotrophic, dispersive larvae that are morphologically similar to their type III siblings, but ecologically similar to Type I planktotrophic larvae. We propose that poecilogony evolved through sequence heterochrony in morphogenesis with accelerated onset of juvenile traits in type III offspring. In addition, we suggest that heterochrony in life-history events (hatching, metamorphosis) also occurs, thereby generating offspring that are dimorphic in both phenotype and ecotype. Over time, selection acting on different levels of ontogeny (morphogenesis vs. dispersal) may balance this polymorphism and allow poecilogony to persist.     

Contrasting patterns of population structure and demographic history in cryptic species of Bostrychia intricata (Rhodomelaceae,Rhodophyta) from New Zealand

Spatial patterns of genetic diversity provide insight into the demography and history of species. Morphologically similar but genetically distinct “cryptic” species are increasingly being recognized in marine organisms through molecular analyses. Such species are, on closer inspection, often discovered to display contrasting life histories or occasionally minor morphological differences; molecular tools can thus be useful indicators of diversity. Bostrychia intricata, a marine red alga, is widely distributed throughout the Southern Hemisphere and comprises many cryptic species. We used mitochondrial cytochrome c oxidase I gene sequences to assess the genetic variation, population genetic structure, and demographic history of B. intricata in New Zealand. Our results supported the existence of three cryptic species of B. intricata (N2, N4, and N5) in New Zealand. Cryptic species N4, which was found throughout New Zealand, showed a higher genetic diversity and wider distribution than the other two species, which were only found in the North Island and northern South Island. Our analyses showed low to moderate genetic differentiation among eastern North Island populations for cryptic species N2, but high differentiation among North and South Island populations for N4, suggesting different population structure between these cryptic species. Data also indicated that N2 has recently undergone population expansion, probably since the Last Glacial Maximum (LGM), while the higher genetic diversity in N4 populations suggests persistence in situ through the LGM. The contrasting population structures and inferred demographic histories of these species highlight that life history can vary greatly even among morphologically indistinguishable taxa.