Acrosome reaction is subfamily specific in sea star fertilization

Species-specific mechanisms during fertilization

The perpetuation and preservation of distinct species rely on mechanisms that ensure that only interactions between gametes of the same species can give rise to viable and fertile offspring. Species-specificity can act at various stages, ranging from physical/behavioral pre-copulatory mechanisms, to pre-zygotic incompatibility during fertilization, to post-zygotic hybrid incompatibility. Herein, we focus on our current knowledge of the molecular mechanisms responsible for species-specificity during fertilization. While still poorly understood, decades of research have led to the discovery of molecules implicated in species-specific gamete interactions, starting from initial sperm-egg attraction to the binding of sperm and egg. While many of these molecules have been described as species-specific in their mode of action, relatively few have been demonstrated as such with definitive evidence. Thus, we also raise remaining questions that need to be addressed in order to characterize gamete interaction molecules as species-specific.

The Molecular Mechanisms of Gametic Incompatibility in Invertebrates

Fertilization (gamete fusion followed by zygote formation) is a multistage process. Each stage is mediated by ligand-receptor recognition of gamete interaction molecules. This recognition includes the movement of sperm in the gradient of egg chemoattractants, destruction of the egg envelope by acrosomal proteins, etc. Gametic incompatibility is one of the mechanisms of reproductive isolation. It is based on species-specific molecular interactions that prevent heterospecific fertilization. Although gametic incompatibility may occur in any sexually reproducing organism, it has been studied only in a few model species. Gamete interactions in different taxa involve generally similar processes, but they often employ non-homologous molecules. Gamete recognition proteins evolve rapidly, like immunity proteins, and include many taxon-specific families. In fact, recently appeared proteins particularly contribute to reproductive isolation via gametic incompatibility. Thus, we can assume a multiple, independent origin of this type of reproductive isolation throughout animal evolution. Gametic incompatibility can be achieved at any fertilization stage and entails different consequences at different taxonomic levels and ranges, from complete incompatibility between closely related species to partial incompatibility between distantly related taxa.

Spermatogenesis as a tool for staging gonad development in the gonochoric appendicularian Oikopleura dioica Fol 1872

Oikopleura dioica, the only gonochoric species among appendicularians, has a spematozoon with a mid-piece and a conspicuous acrosome that, during fertilisation, undergoes a reaction forming an acrosomal process. To provide more insight into the spermatogenesis of a holoplanktonic tunicate species that completes its life cycle in three to five days, changes in the testis during individual growth have been examined. Spermatogenesis has been subdivided into seven stages based on ultrastructural features during the formation and organisation of the male gonad and the relationships between its macroscopic anatomy and the events of sperm differentiation. Gametes undergo highly synchronised differentiation due to the presence of widespread syncytial structures. Both meiosis and spermiogenesis are brief, and the passage from spermatocytes to spermatids involves a progressive segregation of the germ cells from the syncytial mass with the formation of large cytoplasmic bridges and volume reduction for nucleus compacting and cytoplasmic material changing. The nucleus is small and penetrated anteriorly by a complex acrosome and posteriorly by the distal centriole and part of the flagellum. In spermatids, the single, large mitochondrion appears laterally to the nucleus, and finally, in spermatozoa, it migrates into the mid-piece, wrapping the proximal portion of the axoneme. Because this mitochondrial position is reached only in the late phases of spermatogenesis, it suggests that appendicularians have derived oligopyrenic sperms in which the small nucleus results from adaptation to the assembly of numerous spermatozoa inside the narrow space of the testis compacted in the genital cavity. The formulation of a staging system of gonad development in a model tunicate species known for having the most compacted genome in chordates led to a comparison of histological observations with recent molecular data, improving the characterisation of its biology and life cycle in light of evolutionary implications.

Positive selection on sperm ion channels in a brooding brittle star: consequence of life-history traits evolution

Closely related species are key models to investigate mechanisms leading to reproductive isolation and early stages of diversification, also at the genomic level. The brittle star cryptic species complex Ophioderma longicauda encompasses the sympatric broadcast-spawning species C3 and the internal brooding species C5. Here, we used de novo transcriptome sequencing and assembly in two closely related species displaying contrasting reproductive modes to compare their genetic diversity and to investigate the role of natural selection in reproductive isolation. We reconstructed 20 146 and 22 123 genes for C3 and C5, respectively, and characterized a set of 12 229 orthologs. Genetic diversity was 1.5-2 times higher in C3 compared to C5, confirming that species with low parental investment display higher levels of genetic diversity. Forty-eight genes were the targets of positive diversifying selection during the evolution of the two species. Notably, two genes (NHE and TetraKCNG) are sperm-specific ion channels involved in sperm motility. Ancestral sequence reconstructions show that natural selection targeted the two genes in the brooding species. This may result from an adaptation to the novel environmental conditions surrounding sperm in the brooding species, either directly affecting sperm or via an increase in male/female conflict. This phenomenon could have promoted prezygotic reproductive isolation between C3 and C5. Finally, the sperm receptors to egg chemoattractants differed between C3 and C5 in the ligand-binding region. We propose that mechanisms of species-specific gamete recognition in brittle stars occur during sperm chemotaxis (sperm attraction towards the eggs), contrary to other marine invertebrates where prezygotic barriers to interspecific hybridization typically occur before sperm-egg fusion.

Sea star populations diverge by positive selection at a sperm-egg compatibility locus

Fertilization proteins of marine broadcast spawning species often show signals of positive selection. Among geographically isolated populations, positive selection within populations can lead to differences between them, and may result in reproductive isolation upon secondary contact. Here, we test for positive selection in the reproductive compatibility locus, bindin, in two populations of a sea star on either side of a phylogeographic break. We find evidence for positive selection at codon sites in both populations, which are under neutral or purifying selection in the reciprocal population. The signal of positive selection is stronger and more robust in the population where effective population size is larger and bindin diversity is greater. In addition, we find high variation in coding sequence length caused by large indels at two repetitive domains within the gene, with greater length diversity in the larger population. These findings provide evidence of population-divergent positive selection in a fertilization compatibility locus, and suggest that sexual selection can lead to reproductive divergence between conspecific marine populations.

Identification of guanylate cyclases and related signaling proteins in sperm tail from sea stars by mass spectrometry

Marine invertebrates employ external fertilization to take the advantages of sexual reproduction as one of excellent survival strategies. To prevent mismatching, successful fertilization can be made only after going though strictly defined steps in the fertilization. In sea stars, the fertilization process starts with the chemotaxis of sperm followed by hyperactivation of sperm upon arriving onto the egg coat, and then sperm penetrate to the egg coat before achieving the fusion. To investigate whether the initiation of chemotaxis and the following signaling has species specificity, we conducted comparative studies in the protein level among sea stars, Asterias amurensis, A. forbesi, and Asterina pectinifera. Since transcription of messenger ribonucleic acid (mRNA) has been suppressed in gamete, the roles of sperm proteins during the fertilization cannot be investigated by examining the mRNA profile. Therefore, proteomics analysis by mass spectrometry was used in this study. In sea stars, upon receiving asteroidal sperm-activating peptide (asterosap), the receptor membrane-bound guanylate cyclases in the sperm tail trigger sperm chemotaxis. We confirmed the presence of membrane-bound guanylate cyclases in the three sea star species, and they all had the same structural domains including the extracellular domain, kinase-like domain, and guanylate cyclase domain. The majority of peptides recovered were from alpha-helices distributed on the solvent side of the protein. More peptides were recovered from the intracellular domains. The transmembrane domain has not been recovered. The functions of the receptors seemed to be conserved among the species. Furthermore, we identified proteins that may be involved in the guanylate cyclase-triggered signaling pathway.

Conserved sequences of sperm-activating peptide and its receptor throughout evolution, despite speciation in the sea star Asterias amurensis and closely related species

The asteroidal sperm-activating peptides (asterosaps) from the egg jelly bind to their sperm receptor, a membrane-bound guanylate cyclase, on the tail to activate sperm in sea stars. Asterosaps are produced as single peptides and then cleaved into shorter peptides. Sperm activation is followed by the acrosome reaction, which is subfamily specific. In order to investigate the molecular details of the asterosap-receptor interaction, corresponding cDNAs have been cloned, sequenced and analysed from the Asteriinae subfamily including Asterias amurensis, A. rubens, A. forbesi and Aphelasterias japonica, as well as Distolasterias nipon from the Coscinasteriinae subfamily. Averages of 29% and 86% identity were found from the deduced amino acid sequences in asterosap and its receptor extracellular domains, respectively, across all species examined. The phylogenic tree topology for asterosap and its receptor was similar to that of the mitochondrial cytochrome c oxidase subunit I. In spite of a certain homology, the amino acid sequences exhibited speciation. Conservation was found in the asterosap residues involved in disulphide bonding and proteinase-cleaving sites. Conversely, similarities were detected between potential asterosap-binding sites and the structure of the atrial natriuretic peptide receptor. Although the sperm-activating peptide and its receptor share certain common sequences, they may serve as barriers that ensure speciation in the sea star A. amurensis and closely related species.

INTROGRESSION VERSUS IMMIGRATION IN HYBRIDIZING HIGH-DISPERSAL ECHINODERMS

Phylogeographic studies designed to estimate rates and patterns of genetic differentiation within species often reveal unexpected and graphically striking cases of allele or haplotype sharing between species (introgression) via hybridization and backcrossing. Does introgression between species significantly influence population genetic structure relative to more conventional sources of differentiation (drift) and similarity (dispersal) among populations within species? Here we use mtDNA sequences from four species in two genera of sea urchins and sea stars to quantify the relative magnitude of gene flow across oceans and across species boundaries in the context of the trans-Arctic interchange of marine organisms between the Pacific and Atlantic oceans. In spite of the much smaller distances between sympatric congeners, rates of gene flow between sympatric species via heterospecific gamete interactions were small and significantly lower than gene flow across oceans via dispersal of planktonic larvae. We conclude that, in these cases at least, larvae are more effective than gametes as vectors of gene flow.