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.     

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 Polydora cornuta complex (Annelida: Polychaeta) contains populations that are reproductively isolated and genetically distinct

Abstract. We have collected evidence that the nominal species, Polydora cornuta B osc 1802, contains at least three separate species in North America. Specimens of P. cornuta were collected in California, Florida, and Maine, raised in the laboratory, and assessed for reproductive compatibility, genetic similarity, gamete characteristics, and developmental rates. Reproductive crosses between each combination of sex and population revealed variable levels of hybridization at the level of fertilization. Percent fertilization was very low for all combinations (0–7%) except for California females crossed with Florida males (42%). In all interpopulation crosses, fertilized eggs arrested in cleavage and no viable larvae were produced. All pairwise comparisons of the studied populations showed significant differences in multiple reproductive traits. Mitochondrial cytochrome oxidase subunit I DNA sequences revealed large differences between Florida and California worms with a maximum likelihood genetic distance of d =0.860, while Florida and Maine worms were d =0.806, and California and Maine d =0.156. California and New Zealand worms were very similar genetically ( d =0.010). These data strongly suggest that populations of P. cornuta in North America comprise a cryptic species complex composed of at least three distinct lineages.     

Poecilogony in the caenogastropod Calyptraea lichen (Mollusca: Gastropoda)

In marine invertebrates, polymorphism and polyphenism in mode of development are known as “poecilogony.” Understanding the environmental correlates of poecilogony and the developmental mechanisms that produce it could contribute to a better understanding of evolutionary transitions in mode of development. However, poecilogony is rare in marine invertebrates, with only ten recognized, well‐documented cases. Five examples occur in sacoglossan gastropods, and five occur in spionid polychaetes. Here, we document the eleventh case, and the first in a caenogastropod mollusc. Females of Calyptraea lichen collected in the field or reared in the laboratory often produce broods of planktotrophic larvae. They can also be collected with mixed broods, in which each capsule contains planktotrophic larvae, nurse embryos, and adelphophagic embryos. Adelphophages eat the nurse embryos and hatch as short‐lived lecithotrophic larvae, or even as juveniles. Mitochondrial COI and 16S DNA sequences for females with different types of broods differ by less than 0.5%, supporting conspecific status. Some females collected in the field with mixed broods subsequently produced planktotrophic broods, demonstrating that females can produce two different kinds of broods. Calyptraea lichen is therefore polyphenic in two ways: mode of development can vary among embryos within a capsule, and females can change the types of broods they produce.     

The effects of nurse eggs and sibling interactions on the larval development of the poecilogonous annelid Boccardia proboscidea (Spionidae)

In poecilogony, different types of larvae are produced within the same species. Previous studies have suggested maternal control of the production of larval types; however, it is not clear which factors or mechanisms generate contrasting developmental patterns among siblings. The spionid polychaete Boccardia proboscidea produces within the same capsule adelphophagic larvae that eat nurse eggs and siblings and complete all or most of their development inside the capsule (Type A larvae), and larvae with little growth until they hatch as planktotrophic larvae (Type B larvae). In this study, we manipulated capsule content to explore the factors determining larval type in B. proboscidea and the role of extra‐embryonic maternal nutrition and sib–sib interaction in the developmental fate of offspring. When early larval stages were grown individually in vitro, with nurse eggs as the only food source, some of them remained small, while others continue developing into larger pre‐competent larvae by feeding on nurse eggs. This suggests that larval types in B. proboscidea are determined very early in development and are not solely the product of sib–sib interaction inside the capsule. However, our data also suggest that hatching size variability within larval types of a clutch depends on nurse egg availability. Type B larvae grew normally to metamorphosis when phytoplankton was available, but suffered high rates of cannibalism by Type A larvae. These results are consistent with the hypothesis that individual larval fates are determined very early in development and that once their fate is determined, hatching size and intracapsular survival are affected by maternal food provisioning and sibling interaction.     

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.     

Observations on the Cladocera of Lake George, Uganda

Seven species of Cladocera were recorded from the shallow, equatorial Lake George in Uganda, during the period 1967–72. Of these only three, Moina micrura Kurz, Ceriodaphnia cornuta Sars and Daphnia barbata Weltner, are found in the open water and they form a small proportion of the total zooplankton biomass, which is dominated by a cyclopoid copepod. All three species are found throughout the year and carry parthenogenetic eggs at all times. They are smaller species than those predominating in some other East African lakes and some comparisons are also made between C. cornuta and four other species of that genus studied in Great Britain. The mean body lengths and clutch sizes of ovigerous females of C. cornuta vary little through the year. This is, presumably, a reflection of the relatively constant environment in Lake George. Alona poppei is only found in the open water intermittently and is thought to be swept from the peripheral swamps during heavy rains.
The general scarcity of Cladocera in Lake George is thought to be due, either to a shortage of food particles of suitably small size in the phytoplankton, which is dominated by large blue-green algae, or to predation. Although Cladocera are certainly eaten by fish, quantitative data, with which to substantiate either of these hypotheses, is lacking.     

Precopulatory mate guarding and mating behaviour in the rotifer Epiphanes senta (Monogononta: Rotifera)

Epiphanes senta is a littoral rotifer species that occurs in temporary waters and displays a mating behaviour which has not, to my knowledge, so far been described for monogonont rotifers. Monogonont rotifers show distinctive periods within their life cycle during which mictic females appear. Mictic females produce haploid eggs that develop into males or into diapausing eggs if fertilized. The females of E. senta are mostly stationary on the substrate while males are more active swimmers. If they encounter eggs with female embryos of their own species, they attend them and mate with the hatching female. Experiments showed that males are able to discriminate between male, female and diapausing eggs. They exhibit a strong preference for female eggs that are only a few hours away from hatching compared with eggs in early developmental stages. Further experiments did not show any significant differences in male attendance of mictic and amictic eggs. It is hypothesized that males judge the age of a female egg by sensing a chemical that is produced by the growing embryo and diffuses through the egg shell. The male mating behaviour is similar to precopulatory mate guarding known from arthropods but it lacks the monopolization of the female by the male.     

Maternal age and egg weight affect offspring performance in the predatory stink bug Podisus nigrispinus

To investigate influences of maternal age and egg weight on developmental time and body weight of offspring in the Neotropical pentatomid Podisus nigrispinus, a study was carried out using large (0.36 mg) and small (0.32 mg) eggs of young (two-week- old) females and large eggs of old (seven-week-old) females. Incubation period of eggs was influenced only by egg weight, i.e. small eggs took a relatively shorter time to hatch. Both maternal age and egg weight had significant effects on total nymphal and egg-to- adult developmental times. Overall, offspring of young parents had shorter developmental times. None of the developmental periods were significantly affected by sex. No significant differences were found among various offspring groups for percentages of egg hatch, nymphal survival, and sex ratio. Variation of egg weight in relation to the number of eggs per day observed within the first month of life suggests a trade-off between weight of individual eggs and oviposition rate. It is concluded that for mass production of the predators in view of their use in augmentative biocontrol programmes, young parents (2–4 weeks old) should be preferred.     

Reproductive state and care giving in Stegodyphus (Araneae: Eresidae) and the implications for the evolution of sociality

Cooperative brood care is a rare phenomenon in spiders and is restricted to a few social species, including three in the genus Stegodyphus. Brood care in Stegodyphus begins with regurgitation feeding followed by matriphagy: the young consume the body fluids of their mother causing her to die quickly. Whether such an extreme form of maternal care can become a communal task should depend on physiological or historical preconditions. I investigated whether femaleStegodyphus lineatus feed young or allow matriphagy according to their own reproductive state. Broods of young of two age classes (2 or 10 days after hatching) were isolated or fostered out to adult females that were unmated, had eggs or had young. Growth and survival of females and broods were followed over 21 days. The timing of matriphagy depended on the interaction between age of young and state of the foster mother. All broods that were fostered out to females with young grew and survived. Two-day-old young did not survive when isolated or fostered out to unmated females, but some survived and gained weight when placed with foster mothers that cared for egg sacs. Young of 10 days of age grew when fostered out to females with eggs but did not grow or lost weight when isolated or fostered out to unmated females. Survival among 10-day spiderlings was relatively high in all groups but differed significantly between treatments (young isolated or fostered out to unmated females or females with eggs) and control (left with the mother). The results show that these spiders will care for young from other females only when they are in the right developmental state. Such a constraint can have important consequences for the evolution of allomaternal care in social species: unless such a mechanism is overcome, nonreproductives cannot help in brood care. Copyright 2002 The Association for the Study of Animal Behaviour. Published by Elsevier Science Ltd. All rights reserved.     

THE REPRODUCTIVE BIOLOGY OF THE AMPULLARIID SNAIL POMACEA URCEUS (MULLER)

Investigations of the reproductive biology, life cycle and populationdynamics have been undertaken to assess the potential of Pomaceaurceus as a culture species in Trinidad. The species is dioeciousand evidence from microscopic gonad analyses, together withmaturity indices and size frequency distributions in the population,all indicate that the reproductive cycle is annual. Adults spawnat the end of the rainy season months. Hatching and early developmentof the young occur in the dry season (January to May) whileadult females aestivate. The total developmental period variedbetween 22 to 30 days. Mean fecundity is 54 eggs/female witha range of 21 to 93 for egg masses deposited in the field andthose produced under laboratory conditions. ^*Present address: Zoology Dept., University of Aberdeen, TillydroneAve., Aberdeen AB9 2TN. Scotland. (Received 13 January 1988; accepted 11 April 1988)     

Larval development and metamorphosis in Pleurobranchaea maculata, with a review of development in the notaspidea (Opisthobranchia)

Pleurobranchaea maculata is a carnivorous notaspidean that is common in New Zealand. This species produces small eggs (diameter 100 microm) and planktotrophic veligers that hatch in 8 d and are planktonic for 3 weeks before settling on biofilmed surfaces (14 degrees C). Larval development is known in detail for only two other notaspidean species, P. japonica and Berthellina citrina. In all three species of pleurobranchids, mantle and shell growth show striking differences from veligers of other opisthobranch taxa. In young veligers of pleurobranchids, the shell is overgrown by the mantle, new shell is added by cells other than those of the mantle fold, and an operculum does not form. Thus some "adult" traits (e.g., notum differentiation, mechanism of shell growth, lack of operculum) are expressed early in larval development. This suggests that apomorphies characteristic of adult pleurobranchids evolved through heterochrony, with expression in larvae of traits typical of adults of other clades. The protoconch is dissolved post-settlement and not cast off as occurs in other opisthobranch orders, indicating that shell loss is apomorphic. P. maculata veligers are atypical of opisthobranchs in having a field of highly folded cells on the lower velar surface, a mouth that is posterior to the metatroch, and a richly glandular, possibly chemodefensive mantle. These data indicate that notaspidean larvae are highly derived in terms of the novel traits and the timing of morphogenic events. Phylogenetic analysis must consider embryological origins before assuming homology, as morphological similarities (e.g., shell loss) may have developed through distinct mechanisms.     

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.     

Patterns of ionic current through Drosophila follicles and eggs

Large steady electrical currents traverse Drosophila follicles in vitro as well as permeabilized eggs. During the period of main follicle growth (stages 9-11), these currents enter the anterior or nurse cell end of the follicles. This inward current acts like a sodium ion influx with some calcium involvement. During the period of chorion formation (stages 12-14), foci of inward current also appear at the posterior, posterodorsal, and anterodorsal regions of follicles in vitro. In stage 14, the posterior in current acts like a chloride ion efflux. In preblastoderm eggs substantial currents continue to enter their anterior end; while weaker and less frequent ones enter their posterior end. We present models in which the currents during follicle growth are driven by the plasma membrane of the oocyte nurse cell syncitium; the external currents during choriogenesis are driven by the follicular epithelium; while the currents through the preblastoderm egg are driven by its plasma membrane. Measurements of pole-to-pole resistances and voltages across preblastoderm eggs indicate that the transcellular currents normally maintain a steady extracellular voltage gradient along the perivitelline space, with the anterior pole kept negative by perhaps 4 or 5 mV. The developmental significance of these currents is discussed.     

Maternal care in the rockpool amphipod Apherusa jurinei: developmental and environmental cues

There is evidence that active, pre-emergence maternal brood care in amphipod crustaceans may be associated with ‘harsh’ environmental conditions. We examined, in the rockpool amphipod Apherusa jurinei, behavioural activities that may function as a form of active brood care. Only ovigerous females showed ‘curl’ and ‘stretch’ activities, with consequent flushing of the brood pouch and cycling of the eggs therein. There was a significant decline in these activities as embryonic development advanced and brood care almost ceased when well-developed embryos showed a heart pulse and self-ventilation. We propose that this pattern of brood care reflects changes in the physiological requirements of embryos as they develop within the egg membrane. In addition, ovigerous females showed significantly higher levels of brood care under lowered oxygen conditions. They achieved this by increasing the average duration of the ‘stretch’ component, with other brood care components remaining constant. Thus, developmental and environmental cues alter the components of active brood care in distinct ways. Experimental removal showed that the physical presence of eggs in the brood pouch is important in controlling the expression of brood care activities. However, females with all of their eggs removed continued to brood at low levels, suggesting that a maternal state also controls brood care. The sophisticated expression of active maternal brood care in amphipods under ‘harsh’ environmental conditions such as rockpools has implications both for individual reproductive success and the distribution and abundance of brooding versus nonbrooding species. Copyright 2002 Published by Elsevier Science Ltd on behalf of The Association for the Study of Animal Behaviour     

Energy source utilization by embryos and larvae of the muricid snail Chorus giganteus (Lesson, 1829)

The muricacean snail Chorus giganteus presents intracapsular development and the occurrence of nurse eggs that are ingested by the early encapsulated embryos indicate both that these snails develop through a lecitotrophic type of development and that reserves would be sufficient to support settlement and metamorphosis. In order to get more information about the use of energy resources, the dynamics of biochemical components throughout development at three temperatures (9, 12 and 15 °C) and the energetic cost of free-swimming life and metamorphosis are described. The uptake of ³H-alanine, as representative of dissolved organic matter, by embryo and larval stages is also investigated. While protein levels increased at all temperature conditions after ingestion of nurse eggs, lipids only increased when embryo and larvae were reared at 15 °C, and no change in carbohydrate levels was detected at any of the temperatures. The RNA/DNA indexes showed no significant differences with temperature at any stage of development but decreased along with the development of individuals. After hatching, organic matter and energy content of juveniles steadily decreased. Individuals at any of the developmental stage showed to be able to uptake alanine from seawater; the aminoacid uptake capacity increased along with intracapsular development. Uptake of alanine showed to be an active process and to follow Michaelis-Menten kinetics. This would be the first report about dissolved organic matter uptake by encapsulated development stages of any marine invertebrate species and let conclude that these larvae have the ability to obtain exogenous food in a dissolved form and to incorporate it into metabolizable compounds.     

Maternal Care and Infanticide by Males in Helvibis longicauda (Araneae: Theridiidae)

Maternal care in spiders varies from just the construction of a protective silken structure for the eggs and the selection of a safe site to place them, to a long period of association between the mother and spiderlings. Such extended care may involve the active protection from predators and parasitoids, food regurgitation, the production of trophic eggs and even matriphagy. In this study, we describe extended maternal care in Helvibis longicauda (Theridiidae) and evaluate the effectiveness of maternal protection against predators of eggs and spiderlings. We conducted experiments comparing the frequency of egg sac destruction and mortality of spiderlings in the presence and absence of mothers. We also observed the behaviour of the mother and spiderlings during prey capture events and interactions with possible predators. Helvibis longicauda females guard their egg sacs until the emergence of the young and guard the spiderlings for several instar stages, fighting possible predators, including conspecifics. We found that aggressive behaviour by females increased the survival of both eggs and spiderlings in our experiments. Intruder males were the main source of mortality in the absence of females. The benefits of maternal care for the young also include the acquisition of prey items that are captured, immobilized and pre‐digested by the mother. Effective maternal protection and the extended period of supplying food to juveniles probably contribute to the late dispersal of offspring in H. longicauda.     

Ultrastructure of the eggshell and its formation in Planipapillus mundus (Onychophora: Peripatopsidae)

Although the majority of onychophorans are viviparous or ovoviviparous, oviparity has been described in a number of species found exclusively in Australia and New Zealand. Light microscopy and scanning and transmission electron microscopy were used to examine developing eggs and the reproductive tract of the oviparous Planipapillus mundus. Deposited eggs and fully developed eggs dissected from the terminal end of the uteri have an outer thick, slightly opaque chorion, and an inner thin, transparent vitelline membrane. The chorion comprises an outermost extrachorion, sculptured with domes equally spaced over the surface; a middle exochorion, with pores occurring in a pattern of distribution equivalent to that of the domes of the extrachorion above; and an innermost, thick endochorion consisting of a spongelike reticulum of cavities comparable to the respiratory network found in insect eggs. The vitelline membrane lies beneath the chorion, from which it is separated by a fluid‐filled space. The vitelline membrane tightly invests the developing egg. Examination of oocytes in the ovary and developing eggs at various stages of passage through the uterus indicate that the majority of chorion deposition occurs in the midregion of the uterus, where vast networks of endoplasmic reticulum are present in the columnar epithelium. The vitelline membrane, however, is believed to begin its development as a primary egg membrane, surrounding the developing oocytes in the ovary. The vitelline membrane is transformed after fertilization, presumably by secretions from the anterior region of the uterus; hence, it should be more accurately referred to as a fertilization membrane. Aspects of the reproductive biology of P. mundus are also included. J. Morphol., 2008. © 2008 Wiley‐Liss, Inc.