Evolution of egg coats: linking molecular biology and ecology

Abstract

One central goal of evolutionary biology is to explain how biological diversity emerges and is maintained in nature. Given the complexity of the phenotype and the multifaceted nature of inheritance, modern evolutionary ecological studies rely heavily on the use of molecular tools. Here, we show how molecular tools help to gain insight into the role of egg coats (i.e. the extracellular structures surrounding eggs and embryos) in evolutionary diversification. Egg coats are maternally derived structures that have many biological functions from mediating fertilization to protecting the embryo from environmental hazards. They show great molecular, structural and functional diversity across species, but intraspecific variability and the role of ecology in egg coat evolution have largely been overlooked. Given that much of the variation that influences egg coat function is ultimately determined by their molecular phenotype, cutting-edge molecular tools (e.g. proteomics, glycomics and transcriptomics), combined with functional assays, are needed for rigorous inferences on their evolutionary ecology. Here, we identify key research areas and highlight emerging molecular techniques that can increase our understanding of the role of egg coats in the evolution of biological diversity, from adaptation to speciation.

Persistence of an egg mass polymorphism in Ambystoma maculatum: differential performance under high and low nutrients

“…Although reproductive isolation often evolves in polymorphic species, the outer egg jelly layers likely play greater roles in regulating the interactions between the embryo and the environment and not sperm‐egg interactions, thus allowing them to be more variable and responsive to the pressures of natural selection than inner layers (Shu et al. ). This is likely a contributing factor to why we see strong environmental performance differences between the A. maculatum egg mass morphs, yet co‐occurrence and likely interbreeding of the morphs (Ruth et al.…”

“…Whereas egg coats play dual roles mediating both ecological and sperm‐egg interactions, evolution of layers mediating organism‐environment interactions can occur faster by natural selection than layers primarily regulating sperm‐egg interactions due to the need to maintain essential reproductive functions associated with fertilization (Shu et al. ). Reproductive isolation can occur through adaptive divergence when there is strong, differential selection on egg coats in different environments (Nosil et al.…”

Molecular phenotyping of maternally mediated parallel adaptive divergence withinRana arvalisandRana temporaria

“…Egg coats are highly variable across taxa and have important functional roles in sperm–egg interactions as well as prehatching and hatching‐related performance of offspring (Wong & Wessel ; Menkhorst & Selwood ; Shu et al . ). Egg coats can act as a species barrier and have been intensively studied in the context of speciation, especially in free‐spawning marine taxa (Swanson and Vacquier ; Palumbi ; Vacquier and Swanson ; Hart et al .…”

Microanatomical diversification of the zona pellucida in aplochelioid killifishes

“…For an extensive review of egg coats see Shu et al . (). In fishes, the ZP's secondary egg layer often contains structural features such as filaments, protuberances, jelly coatings or honeycomb‐like patterns that help the egg adhere to the substratum (Riehl & Patzner, ).…”