Fine-scale genetic structure in a salamander with two reproductive modes: Does reproductive mode affect dispersal?

Reproduction is intimately linked with dispersal, but the effects of changes in reproductive strategies on dispersal have received little attention. Such changes have occurred in many taxonomic groups, resulting in profound alterations in life-history. In amphibians, many species shifted from oviparous/larviparous aquatic reproduction (deposition of eggs or pre-metamorphic larvae in water) to pueriparous terrestrial reproduction (parturition of terrestrial juveniles). The latter provides greater independence from water by skipping the aquatic larval stage; however, the eco-evolutionary implications of this evolutionary step have been underexplored, largely because reproductive modes rarely vary at the intraspecific level, preventing meaningful comparisons. We studied the effects of a transition to pueriparity on dispersal and fine-scale genetic structure in the fire salamander (Salamandra salamandra), a species exhibiting two co-occurring reproductive modes: larviparity and pueriparity. We performed genetic analyses (parentage and genetic spatial autocorrelation) using 11 microsatellite loci to compare dispersal and fine-scale genetic structure in three larviparous and three pueriparous populations (354 individuals in total). We did not find significant differences between reproductive modes, but in some larviparous populations movement patterns may be influenced by site-specific features (type of water bodies), possibly due to passive water-borne dispersal of larvae along streams. Additionally, females (especially larviparous ones) appeared to be more philopatric, while males showed greater variation in dispersal distances. This study also points to future avenues of research to better understand the eco-evolutionary implications of changes in reproductive modes in amphibians.     

Life-history responses of larviparous Boettcherisca formosensis (Diptera: Sarcophagidae) to larval competition for food, including comparisons with oviparous Hemipyrellia ligurriens (Calliphoridae)

Abstract. 1 The effect of larval rearing density on life-history parameters of Boettcherisca formosensis Kirner & Lopes (Sarcophagidae) was investigated. Increases in rearing density resulted in lowered larval survivorship, shortened larval development time and production of smaller, shorter-lived adults with reduced fecundity.
2. B. formosensis is larviparous. Average brood size was 17.5±1.0 (mean±M) larvae, which was much less than the average number of mature larvae inside gravid females. Females apparently produced a series of small broods, distributing their offspring over a number of carcasses.
3. Compared with the oviparous species Hemipyrellia ligurriens (Wiedemann) (Calliphoridae), B. formosensis adults were larger and longer-lived, with a longer larval development time but shorter larval feeding period. However, females had a shorter pre-reproductive period, were less fecund, and had a lower life time reproductive investment.
4. B. formosensis had lower relative performance (measured by the composite index of performance, r') than H. ligurriens over the larval rearing density range, and was more sensitive to increases in density. Although the r' values suggest that the sarcophagid may be a competitively inferior species, other features which are not included in the index (such as larvipary, short larval feeding period and spreading of offspring from a single brood among carcasses) may be of significant adaptive value to B. formosensis.     

THE GENETIC BASIS OF LIFE-HISTORY CHARACTERS IN A POLYCHAETE EXHIBITING PLANKTOTROPHY AND LECITHOTROPHY

The polychaete Streblospio benedicti is unusual in that several field populations consist of individuals that exhibit either planktotrophic or lecithotrophic larval development. Planktotrophy in this species involves production of many small ova that develop into feeding larvae with a two- to three-week planktonic period. Lecithotrophy involves production of fewer, larger ova that develop into nonfeeding larvae that are brooded longer and have a brief planktonic stage. Reciprocal matings were performed to investigate genetic variance components and the correlation structure of life-history traits associated with planktotrophy and lecithotrophy. Our objective was to better understand persistence of this developmental dichotomy in Streblospio benedicti, and among marine invertebrates in general. Substantial additive genetic variation (75–98% of total) was detected for the following characters at first reproduction: female length; position of the first gametogenic setiger and first brood pouch; ovum diameter; three traits related to fecundity (numbers of ova per ovary, larvae per brood pouch, and larvae per brood); median larval planktonic period and the presence of larval swimming setae; but not for total number of brood pouches; larval length; larval feeding; and larval survivorship. Based on the unusual geographic distribution of development modes in this species, we hypothesize that the developmental traits have evolved in allopatry and have only recently come into contact in North Carolina. The high additive contribution to variance observed for many traits may be inflated due to (a) nonrandom breeding in nature, and (b) examination of only one component of an age-structured population at one time. Nuclear interaction variance and maternal variance accounted for 84% of the total variation in larval survivorship. This observation supports other empirical studies and theoretical predictions that nonadditive components of variance will increase in importance in individual traits that are most closely tied to fitness. A network of life-history trait correlations was observed that defines distinct planktotrophic and lecithotrophic trait complexes. Negative genetic correlations were present between fecundity and egg size, between fecundity and position of the first gametes, and between larval survivorship and median planktonic period. Positive genetic correlations were detected between fecundity and female size at first reproduction and between planktonic period and the presence of swimming setae. Intergenerational product-moment correlations were negative for larval length and fecundity, planktonic period and egg size, female size and larval survivorship, and fecundity and larval survivorship. If the genetic correlation structure observed in the laboratory persists in the field, it may constrain responses of individual characters to directional selection, and indirectly perpetuate the dichotomies associated with planktotrophy and lecithotrophy.     

A REVIEW OF LARVAL DEVELOPMENT IN THE INTERTIDAL LIMPET GENUS SIPHONARIA (GASTROPODA: PULMONATA)

A compilation of distributional and life-history data relatingto mode of larval development is presented for 26 species ofSiphonana, a genus of intertidal pulmonates. Most species depositgelatinous benthic egg masses with two species releasing pelagicegg masses. Fifteen species hatch as planktonic-developing larvae,nine hatch as direct-developing juveniles, and in a furthertwo larvae hatch with both the swimming velar apparatus (associatedwith plank-tonic development) and a crawling foot (associatedwith direct development). Data on mode of larval developmentare interpreted with respect to some adaptive models. Despiteimportant exceptions, there is support for adaptive models basedupon egg capsule size (direct developers hatch from larger eggcapsules) and intertidal distribution (direct developers generallyoccur higher on the shore than planktonic developers). Worldwide,planktonic developers are more widespread than direct-developingspecies, and individual planktonic species have a greater meanlatitudinal range. The evidence for adaptive models relatinglatitudinal distribution to developmental mode is equivocal.There appears to be no clear relationship between body sizeand developmental mode in the genus, although the smallest specieshas direct development and the largest has planktonic development.In most siphonariid subgenera, developmental mode appears tobe constant, but two subgenera contain a mixture of developmentaltypes (Received 1 November 1993; accepted 15 April 1994)     

Comparative landscape genetics reveals the evolution of viviparity reduces genetic connectivity in fire salamanders

Evolutionary changes in reproductive mode may affect co‐evolving traits, such as dispersal, although this subject remains largely underexplored. The shift from aquatic oviparous or larviparous reproduction to terrestrial viviparous reproduction in some amphibians entails skipping the aquatic larval stage and, thus, greater independence from water. Accordingly, amphibians exhibiting terrestrial viviparous reproduction may potentially disperse across a wider variety of suboptimal habitats and increase population connectivity in fragmented landscapes compared to aquatic‐breeding species. We investigated this hypothesis in the fire salamander (Salamandra salamandra), which exhibits both aquatic‐ (larviparity) and terrestrial‐breeding (viviparity) strategies. We genotyped 426 larviparous and 360 viviparous adult salamanders for 13 microsatellite loci and sequenced a mitochondrial marker for 133 larviparous and 119 viviparous individuals to compare population connectivity and landscape resistance to gene flow within a landscape genetics framework. Contrary to our predictions, viviparous populations exhibited greater differentiation and reduced genetic connectivity compared to larviparous populations. Landscape genetic analyses indicate viviparity may be partially responsible for this pattern, as water courses comprised a significant barrier only in viviparous salamanders, probably due to their fully terrestrial life cycle. Agricultural areas and, to a lesser extent, topography also decreased genetic connectivity in both larviparous and viviparous populations. This study is one of very few to explicitly demonstrate the evolution of a derived reproductive mode affects patterns of genetic connectivity. Our findings open avenues for future research to better understand the eco‐evolutionary implications underlying the emergence of terrestrial reproduction in amphibians.     

Population structure is not a simple function of reproductive mode and larval type: insights from tropical corals

1. For a wide range of organisms, heritable variation in life-history characteristics has been shown to be strongly subject to selection, reflecting the impact that variation in characters such as genotypic diversity, duration of larval development and adaptations for dispersal can have on the fitness of offspring and the make-up of populations. Indeed, variation in life-history characteristics, especially reproduction and larval type, have often been used to predict patterns of dispersal and resultant population structures in marine invertebrates. 2. Scleractinian corals are excellent models with which to test this relationship, as they exhibit almost every possible combination of reproductive mode and larval type. Some general patterns are emerging but, contrary to expectations, genetic data suggest that while populations of broadcast spawning species may be genotypically diverse they may be heavily reliant on localized recruitment rather than widespread dispersal of larvae. 3. Here we use microsatellites to test the importance of localized recruitment by comparing the genetic structure of populations of two broadcast spawning corals with contrasting modes of reproduction and larval development; Goniastrea favulus is self-compatible, has sticky, negatively buoyant eggs and larvae and is expected to have restricted dispersal of gametes and larvae. In contrast, Platygyra daedalea is self-incompatibile, spawns positively buoyant egg-sperm bundles and has planktonic development. 4. Surprisingly, spatial-autocorrelation revealed no fine-scale clustering of similar genotypes within sites for G. favulus, but showed a non-random distribution of genotypes in P. daedalea. Both species showed similar levels of genetic subdivision among sites separated by 50-100 m (F(ST) = 0.03), suggesting that larval dispersal may be equivalent in both species. 5. Interestingly, as fragmentation has been considered rare in massive corals, our sample of 284 P. daedalea colonies included 28 replicated genotypes that were each unlikely (P < 0.05) to have been derived independently from sexual reproduction. 6. We conclude that the extreme life history of G. favulus does not produce unusually fine-scale genetic structure and subsequently, that reproductive mode and larval type may not be not good predictors of population structure or dispersal ability.     

Deconstructing environmental predictability: seasonality,environmental colour and the biogeography of marine life histories

Environmental predictability is predicted to shape the evolution of life histories. Two key types of environmental predictability, seasonality and environmental colour, may influence life‐history evolution independently but formal considerations of both and how they relate to life history are exceedingly rare. Here, in a global biogeographical analysis of over 800 marine invertebrates, we explore the relationships between both forms of environmental predictability and three fundamental life‐history traits: location of larval development (aplanktonic vs. planktonic), larval developmental mode (feeding vs. non‐feeding) and offspring size. We found that both dispersal potential and offspring size related to environmental predictability, but the relationships depended on both the environmental factor as well as the type of predictability. Environments that were more seasonal in food availability had a higher prevalence of species with a planktonic larval stage. Future studies should consider both types of environmental predictability as each can strongly affect life‐history evolution.     

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.     

Testing adaptive hypotheses on the evolution of larval life history in acorn and stalked barnacles

Despite strong selective pressure to optimize larval life history in marine environments, there is a wide diversity with regard to developmental mode, size, and time larvae spend in the plankton. In the present study, we assessed if adaptive hypotheses explain the distribution of the larval life history of thoracican barnacles within a strict phylogenetic framework. We collected environmental and larval trait data for 170 species from the literature, and utilized a complete thoracican synthesis tree to account for phylogenetic nonindependence. In accordance with Thorson's rule, the fraction of species with planktonic‐feeding larvae declined with water depth and increased with water temperature, while the fraction of brooding species exhibited the reverse pattern. Species with planktonic‐nonfeeding larvae were overall rare, following no apparent trend. In agreement with the “size advantage” hypothesis proposed by Strathmann in 1977, egg and larval size were closely correlated. Settlement‐competent cypris larvae were larger in cold water, indicative of advantages for large juveniles when growth is slowed. Planktonic larval duration, on the other hand, was uncorrelated to environmental variables. We conclude that different selective pressures appear to shape the evolution of larval life history in barnacles.     

Genetic polymorphism and trade-offs in the early life-history strategy of the Pacific oyster, Crassostrea gigas (Thunberg, 1795): a quantitative genetic study

We investigated genetic variability and genetic correlations in early life-history traits of Crassostrea gigas. Larval survival, larval development rate, size at settlement and metamorphosis success were found to be substantially heritable, whereas larval growth rate and juvenile traits were not. We identified a strong positive genetic correlation between larval development rate and size at settlement, and argue that selection could optimize both age and size at settlement. However, trade-offs, resulting in costs of metamorphosing early and large, were suggested by negative genetic correlations or covariances between larval development rate/size at settlement and both metamorphosis success and juvenile survival. Moreover, size advantage at settlement disappeared with time during the juvenile stage. Finally, we observed no genetic correlations between larval and juvenile stages, implying genetic independence of life-history traits between life-stages. We suggest two possible scenarios for the maintenance of genetic polymorphism in the early life-history strategy of C. gigas.     

Egg Size Effects across Multiple Life-History Stages in the Marine Annelid Hydroides diramphus

The optimal balance of reproductive effort between offspring size and number depends on the fitness of offspring size in a particular environment. The variable environments offspring experience, both among and within life-history stages, are likely to alter the offspring size/fitness relationship and favor different offspring sizes. Hence, the many environments experienced throughout complex life-histories present mothers with a significant challenge to optimally allocate their reproductive effort. In a marine annelid, we tested the relationship between egg size and performance across multiple life-history stages, including: fertilization, larval development, and post-metamorphosis survival and size in the field. We found evidence of conflicting effects of egg size on performance: larger eggs had higher fertilization under sperm-limited conditions, were slightly faster to develop pre-feeding, and were larger post-metamorphosis; however, smaller eggs had higher fertilization when sperm was abundant, and faster planktonic development; and egg size did not affect post-metamorphic survival. The results indicate that egg size effects are conflicting in H. diramphus depending on the environments within and among life-history stages. We suggest that offspring size in this species may be a compromise between the overall costs and benefits of egg sizes in each stage and that performance in any one stage is not maximized.     

Biocomplexity in Populations of European Anchovy in the Adriatic Sea

The sustained exploitation of marine populations requires an understanding of a species'' adaptive seascape so that populations can track environmental changes from short- and long-term climate cycles and from human development. The analysis of the distributions of genetic markers among populations, together with correlates of life-history and environmental variability, can provide insights into the extent of adaptive variation. Here, we examined genetic variability among populations of mature European anchovies (n = 531) in the Adriatic (13 samples) and Tyrrhenian seas (2 samples) with neutral and putative non-neutral microsatellite loci. These genetic markers failed to confirm the occurrence of two anchovy species in the Adriatic Sea, as previously postulated. However, we found fine-scale population structure in the Adriatic, especially in northern areas, that was associated with four of the 13 environmental variables tested. Geographic gradients in sea temperature, salinity and dissolved oxygen appear to drive adaptive differences in spawning time and early larval development among populations. Resolving adaptive seascapes in Adriatic anchovies provides a means to understand mechanisms underpinning local adaptation and a basis for optimizing exploitation strategies for sustainable harvests.     

Variation in adult shell morphology and life-history traits in the land snail Oreohelix cooperi in relation to biotic and abiotic factors

In this study, we explore factors influencing variation in adultshell morphology and life-history characteristics (offspringsize and number) in the ovoviviparous land snail, Oreohelixcooperi, from the Black Hills, South Dakota and Wyoming, USA.To date, no study of shell morphology and life-history characteristicsin ovoviviparous land snails has included a combination of datasetsincluding genetic data, life-history traits, shell morphologyand multiple environmental factors. We report that differencesin shell size among populations are strongly related to meanannual temperature (and the highly correlated variable elevation)and population density (measured as shell density). In addition,shell size varies among populations, despite an apparent lackof population genetic differentiation. Common factors thoughtto influence adult shell size, like precipitation and calciumlevels, do not have a significant effect in this study. Adultsize strongly influences per-clutch reproductive output, withlarger snails having larger and more offspring. As mean annualtemperature and shell density affect adult shell size, theyalso indirectly affect per-clutch output. The results suggestthat a large portion of the life-history variation in O. cooperiis environmentally induced, as has been found in oviparous landsnails and brooding freshwater bivalves. (Received 20 July 2006; accepted 10 January 2007)     

MOLECULAR PHYLOGENETIC ANALYSIS OF LIFE-HISTORY EVOLUTION IN ASTERINID STARFISH

We analyzed phylogenetic relationships among 12 nominal species of starfish in the genera Patiriella and Asterina (Order Valvatida, Family Asterinidae), based on complete sequences for a mitochondrial protein coding gene (cytochrome oxidase subunit I) and five mitochondrial transfer RNA genes (alanine, leucine, asparagine, glutamine, and proline) (1923 bp total). The resulting phylogeny was used to test a series of hypotheses about the evolution of life-history traits. (1) A complex, feeding, planktonic larva is probably ancestral for these starfish, but this is not the most parsimonious reconstruction of ancestral larval states. (2) The feeding larval form was lost at least four times among these species, and three of these losses occurred among members of a single clade. (3) Small adult size evolved before both cases of hermaphroditism and viviparous brooding, but viviparity was not always preceded by an intermediate form of external brooding. (4) An ordered transformation series from feeding planktonic development to viviparous brooding has been predicted for starfish, but we could not find an example of this transformation series. (5) Viviparity evolved recently (< 2 Mya). (6) Both species selection and transformation of lineages may have contributed to the accumulation of species with nonfeeding development among these starfish. (7) Neither Asterina nor Patiriella are monophyletic genera. Larval forms and life-history traits of these starfish have evolved freely under no obvious constraints, contrary to the widely assumed evolutionary conservatism of early development.     

Genetic diversity and species diversity of stream fishes covary across a land-use gradient

Genetic diversity and species diversity are expected to covary according to area and isolation, but may not always covary with environmental heterogeneity. In this study, we examined how patterns of genetic and species diversity in stream fishes correspond to local and regional environmental conditions. To do so, we compared population size, genetic diversity and divergence in central stonerollers (Campostoma anomalum) to measures of species diversity and turnover in stream fish assemblages among similarly sized watersheds across an agriculture–forest land-use gradient in the Little Miami River basin (Ohio, USA). Significant correlations were found in many, but not all, pair-wise comparisons. Allelic richness and species richness were strongly correlated, for example, but diversity measures based on allele frequencies and assemblage structure were not. In-stream conditions related to agricultural land use were identified as significant predictors of genetic diversity and species diversity. Comparisons to population size indicate, however, that genetic diversity and species diversity are not necessarily independent and that variation also corresponds to watershed location and glaciation history in the drainage basin. Our findings demonstrate that genetic diversity and species diversity can covary in stream fish assemblages, and illustrate the potential importance of scaling observations to capture responses to hierarchical environmental variation. More comparisons according to life history variation could further improve understanding of conditions that give rise to parallel variation in genetic diversity and species diversity, which in turn could improve diagnosis of anthropogenic influences on aquatic ecosystems.     

Deconstructing latitudinal species richness patterns in the ocean: does larval development hold the clue?

The consistent decrease in species richness with latitude shows several exceptions among marine organisms. We hypothesize that contrasting latitudinal diversity gradients can be explained by differences in critical life-history attributes, such as mode of larval development (MLD). We deconstructed latitudinal species richness patterns of marine benthic invertebrates according to MLD to elucidate differences in patterns of species richness and to reveal underlying processes. The patterns of species richness were remarkably similar across taxa within MLD but differed between MLD. Species richness decreased polewards in planktotrophic species and increased in direct developers. Temperature explained most of the variation in species richness. Low temperature at high latitudes may generally favour direct developing species, but, together with low chlorophyll- a concentration, limit the distribution of planktotrophic species. The contrasting influence of temperature on different MLDs might be explained by its effect on the length of planktonic life and on brooding costs.     

Spatial and genetic investigation of aggregation in Ischnochiton (Polyplacophora; Neoloricata; Ischnochitonina; Ischnochitonidae; Ischnochitoninae) species with different larval development

Many species are aggregated at some spatial scale but for some species, due to habitat or behaviour, aggregation can be difficult to assess quantitatively. Here, aggregation of chiton species (Polyplacophora; Neoloricata; Ischnochitonina; Ischnochitonidae; Ischnochitoninae; Ischnochiton) living under boulders in intertidal boulder fields was investigated at several relatively small spatial scales. Closely related species were found to be aggregated at the level of individual boulders, but evidence for grouping at the larger scale of patches of boulders varied. When organisms are mobile, dispersal can have an important influence on spatial patterns. Some organisms, such as marine invertebrates, have a highly dispersive larval phase that can influence spatial patterns of adults, particularly where the adult is sessile or relatively sedentary. Aggregation was compared in two species to test whether different modes of larval development influence spatial pattern. There was some evidence that species with a planktonic larva were even more aggregated than expected, in contrast to predictions based on this mode of development. Both types of larval development (planktonic and non‐planktonic) produce larvae with short development times in these species, so one possible explanation for the grouping habit of these chitons is that they do not disperse, at all, from their natal boulder. The complexity of the boulder field habitat and the cryptic behaviour of these chitons may also contribute to a lack of dispersal. A simple application of a genetic method indicated, however, that philopatry at this scale is unlikely.     

Offspring size and survival in the frog Rana latastei: from among-population to within-clutch variation

Egg size is considered to be a major maternal effect for offspring in oviparous organisms. It has profound consequences on fitness, and differences in egg size are viewed as plastic responses to environmental variability. However, it is difficult to identify the effect of egg size per se because egg size can covary with genetic features of the mother and with other nongenetic factors. We analysed the relationship between offspring starting size (i.e. a proxy of egg size) and larval survival in the frog Rana latastei . We analysed this relationship: (1) among five populations at different altitudes; (2) among clutches laid from different females; and (3) among siblings within clutches, to evaluate the effect of starting size. We observed differences among populations for offspring size, but starting size was not related to altitude or genetic diversity. Mortality was higher in populations and families with small average starting size; however, among siblings, the relationship between starting size and mortality was not verified. The relationship observed among clutches may therefore be caused by covariation between egg size and other effects. This suggests that the covariation between egg size and other effects can result in apparent relationships between egg size and fitness-related traits. Proximate and ultimate factors can cause the phenotypic variation of hatchlings in the wild, and key traits can be related to this variation, but the underlying causes require further investigation.  © 2009 The Linnean Society of London, Biological Journal of the Linnean Society , 2009, 97 , 845–853.     

It's about time: divergence, demography, and the evolution of developmental modes in marine invertebrates

Differences in larval developmental mode are predicted to affect ecological and evolutionary processes ranging from gene flow and population bottlenecks to rates of population recovery from anthropogenic disturbance and capacity for local adaptation. The most powerful tests of these predictions use comparisons among species to ask how phylogeographic patterns are correlated with the evolution and loss of prolonged planktonic larval development. An important and largely untested assumption of these studies is that interspecific differences in population genetic structure are mainly caused by differences in dispersal and gene flow (rather than by differences in divergence times among populations or changes in effective population sizes), and that species with similar patterns of spatial genetic variation have similar underlying temporal demographic histories. Teasing apart these temporal and spatial patterns is important for understanding the causes and consequences of evolutionary changes in larval developmental mode. New analytical methods that use the coalescent history of allelic diversity can reveal these temporal patterns, test the strength of traditional population-genetic explanations for variation in spatial structure based on differences in dispersal, and identify strongly supported alternative explanations for spatial structure based on demographic history rather than on gene flow alone. We briefly review some of these recent analytical developments, and show their potential for refining ideas about the correspondence between the evolution of larval developmental mode, population demographic history, and spatial genetic variation.     

Control and Consequences of Alternative Developmental Modes in a Poecilogonous Polychaete

SYNOPSIS. The poecilogonous polychaete Streblospio benedicti(Webster) exhibits both planktotrophic and lecithotrophic modesof larval development. The alternative trophic modes are associatedwith differences in age and size at maturation, offspring number,size and energetic investment, larval planktonic period, morphologyand survivorship. This paper reviews a decade of research intothe control and consequences of the traits associated with planktotrophyand lecithotrophy in S. benedicti. The dominant control on reproductiveand developmental characters is genetic. Significant additivegenetic variance has been detected for egg diameter, fecundity,larval planktonic period and aspects of larval morphology. However,environmental factors such as temperature, food quality andphotoperiod, and intrinsic factors such as maternal age, exertconsiderable influence on non-trophic developmental traits (e.g.,offspring number, size and energy content). Demographic consequencesof development mode are reviewed for field and laboratory demesof S. benedicti dominated by individuals exhibiting either planktotrophyor lecithotrophy. Similar population size structure, fluctuationsin abundance, P: B ratios, and estimated population growth ratesare achieved through trade-offs between survivorship and fecundity. Development mode may best be viewed as a complex set of traitsthat are intimately linked developmentally and evolutionarilyto other aspects of an organism's life history. Greater insightinto the control and consequences of development mode shouldresult from further investigation of these linkages