Viviparity in the Sea StarCryptasterina hystera(Asterinidae)—Conserved and Modified Features in Reproduction and Development

Abstract

Cryptasterina hystera has a highly derived life history with intragonadal development and juveniles that emerge from the parent's reproductive tract. The gonads are ovotestes with developing eggs separated from sperm by follicle cells. C. hystera has typical echinosperm that must enter the gonoduct of conspecifics to achieve fertilization. During oogenesis, an initial period of yolk accumulation is followed by hypertrophic lipid deposition, the major contributor to the increase in egg size. 1-Methyladenine induces egg maturation and ovulation, but the spawning component of the hormonal cascade is suppressed. This is the major alteration in reproduction associated with evolution of viviparity in C. hystera. The switch to viviparity was not accompanied by major change in gonad structure, indicating there were few or no anatomical constraints for evolution of a marsupial function for the gonad. Despite their intragonadal habitat, the brachiolaria are equipped for a planktonic life, swimming in gonadal fluid. During the gastrula stage, lipid provisions are released into the blastocoel where they are stored for juvenile development. The eggs of C. hystera have light and dark cytoplasmic regions that mark animal-vegetal polarity. The dark pigment provided a marker to follow the fate of vegetal cells. Live birth is rare in the Echinodermata and the incidence of this form of brooding in the phylum is reviewed.

Extraordinarily rapid life-history divergence between Cryptasterina sea star species

“…Nucleotide diversity and haplotype diversity are also low for the mitochondrial locus, suggesting that C. hystera has experienced a severe population bottleneck. C. hystera may also be self-fertile [29], and while the lack of heterozygosity is consistent with selfing, it also precludes direct detection of selfing by paternity analysis of broodmates. We suspect that some level of selfing does occur; however, selfing alone would not account for the low levels of genetic diversity at the mtDNA locus.…”

“…However, such arguments do not address the selective causes of the origin of livebearing and other forms of benthic reproduction without larval dispersal in specific lineages. Viviparity, despite its association with limited dispersal ability, has evolved several times across many different types of marine invertebrate taxa, including multiple lineages of Asterinidae [24,29,30,52,54,55], and it is consistently associated with species in colder water temperatures and higher latitudes [53,56,57]. For example, in cowries, the loss of planktonic development seems to be directly tied to environmental factors specifically associated with temperature and productivity [45], suggesting that environmental selection may be playing a key role in the evolutionary changes in life-history traits and associated speciation.…”

“…Individuals of C. hystera are assumed to self-fertilize (and our genetic data are consistent with the expected effects of selfing on genetic diversity). Lacking specialized anatomical structures for sperm transfer or storage, and possessing fully functional larvae, the transition to live-bearing in C. hystera may represent the minimal level of modification for the evolution of live-bearing [24,25,29], i.e. the evolution of internal self-fertilization and the loss of female spawning behaviour.…”

Ultrastructure of hull formation during oogenesis inRhyssoplax tulipa(=Chiton tulipa) (Chitonidae: Chitoninae)

“…Quite similar micro-apocrine secretion processes are well known in the insect literature (Cristofoletti et al, 2001;Bolognesi et al, 2001) and have also been discovered in mice where Hermo and Jacks (2002) remarked on Nature's "ingenuity in bypassing the classical secretory role via apocrine secretion". The apocrine mechanism of secretion was described also in embryogenesis of the sea urchin Heliocidaris erythrogramma and sea star Cryptasterina hystera, where lipids are secreted into the blastocoel taking a fragment of cell membrane (Henry et al, 1991;Byrne, 2005).…”

Selfing in Parvulastra exigua: an asterinid sea star with benthic development

“…Release of sperm onto communal egg masses by aggregated P. exigua and Aquilonastra minor is a form of pseudocopulation (Komatsu et al 1979;Byrne 1992Byrne , 2005. The recently discovered Aquilonastra byrneae may also have this reproductive strategy (Byrne and Walker 2007).…”