Morphological evolution in sea urchin development: hybrids provide insights into the pace of evolution

Modeling Fertilization Outcome in a Changing World

Marine organisms have complex life histories. For broadcast spawners, successful continuation of the population requires their small gametes to make contact in the water column for sufficiently long periods for fertilization to occur. Anthropogenic climate change has been shown to impact fertilization success in various marine invertebrates, including sea urchins, which are key grazers in their habitats. Gamete performance of both sexes declined when exposed to elevated temperatures and/or pCO2 levels. Examples of reduced performance included slower sperm swimming speed and thinning egg jelly coat. However, such responses to climate change stress were not uniform between individuals. Such variations could serve as the basis for selection. Fertilization kinetics have long been modeled as a particle collision process. Here, we present a modified fertilization kinetics model that incorporates individual variations in performance in a more environmentally relevant regime, and which the performance of groups with different traits can be separately tracked in a mixture. Numerical simulations highlight that fertilization outcomes are influenced by changes in gamete traits as they age in sea water and the presence of competition groups (multiple dams or sires). These results highlight the importance of considering multiple individuals and at multiple time points during in vivo assays. We also applied our model to show that interspecific variation in climate stress vulnerabilities elevates the risk of hybridization. By making a numerical model open-source, we aim to help us better understand the fate of organisms in the face of climate change by enabling the community to consider the mean and variance of the response to capture adaptive potential.

Procuring animals and culturing of eggs and embryos

Echinoderms and especially echinoids have a rich history as model systems for the study of oogenesis, fertilization, and early embryogenesis. The ease of collecting and maintaining adults, as well as in obtaining gametes and culturing large quantities of synchronous embryos, is complemented by the ability to do biochemistry, reverse genetics, embryo manipulations and study gene regulatory networks. The diversity of species and developmental modes as well as unparalleled transparency in early developmental stages also makes echinoderms an excellent system in which to study evolutionary aspects of developmental biology. This chapter provides a practical guide to experimental methods for procuring adults and gametes, achieving synchronous in vitro fertilization, and culturing embryos through early larval stages for several echinoderm species representing four classes (Echinoidea, Asteroidea, Ophiuroidea, and Holothuroidea). We provide specific examples of protocols for obtaining adults and gametes and for culturing embryos of a selected number of species for experimental analysis of their development. The species were chosen to provide breadth across the phylum Echinodermata, as well as to provide practical guidelines for handling some of the more commonly studied species. For each species, we highlight specific advantages, and special note is made of key issues to consider when handling adults, collecting gametes, or setting and maintaining embryo cultures. Finally, information regarding interspecific crosses is provided.

Morphology and gene analysis of hybrids between two congeneric sea stars with different modes of development

The sea star Astropecten scoparius has feeding bipinnarian larvae, whereas its congener Astropecten latespinosus has nonfeeding barrel-shaped larvae. To investigate evolutionary changes in the development of asteroids, we performed reciprocal crosses between these two species with different larval forms. In the cross between A. scoparius eggs and A. latespinosus sperm, embryos developed into bipinnaria-like larvae. The larvae exhibited either a functional digestive system (a maternal feature) or a nonfunctional digestive system with the tip of the archenteron not connected to the stomodeum (a paternal characteristic). However, in the reciprocal cross between A. latespinosus eggs and A. scoparius sperm, barrel-shaped larvae resembling those of A. latespinosus were produced, in addition to bipinnaria-like larvae, some with functional digestive systems and some with nonfunctional ones. Juveniles were produced from all types of crosses. 18S rDNA was used as a gene marker in cycle sequencing analysis to investigate the genetic features of these juveniles. The sequences of juveniles from bipinnaria-like larvae showed double-peak nucleotide signals, indicating a biparental genome. On the other hand, juveniles from barrel-shaped larvae from A. latespinosus eggs and A. scoparius sperm showed the same sequence as A. latespinosus juveniles. This suggests that bipinnaria-like larvae of both crosses are always hybrids, whereas barrel-shaped larvae develop parthenogenetically.