Molecular Mechanisms of Spawning Habits for the Adaptive Radiation of Endemic East Asian Cyprinid Fishes

Abstract

Despite the widespread recognition of adaptive radiation as a driver of speciation, the mechanisms by which natural selection generates new species are incompletely understood. The evolutionary radiation of endemic East Asian cyprinids has been proposed as evolving through a change in spawning habits, involving a transition from semibuoyant eggs to adhesive eggs in response to crosslinked river-lake system formation. Here, we investigated the molecular mechanisms that underpin this radiation, associated with egg hydration and adhesiveness. We demonstrated that semibuoyant eggs enhance hydration by increasing the degradation of yolk protein and accumulation of Ca2+ and Mg2+ ions, while adhesive eggs improve adhesiveness and hardness of the egg envelope by producing an adhesive layer and a unique 4th layer to the egg envelope. Based on multiomics analyses and verification tests, we showed that during the process of adaptive radiation, adhesive eggs downregulated the “vitellogenin degradation pathway,” “zinc metalloprotease pathway,” and “ubiquitin-proteasome pathway” and the pathways of Ca2+ and Mg2+ active transport to reduce their hydration. At the same time, adhesive eggs upregulated the crosslinks of microfilament-associated proteins and adhesive-related proteins, the hardening-related proteins of the egg envelope, and the biosynthesis of glycosaminoglycan in the ovary to generate adhesiveness. These findings illustrate the novel molecular mechanisms associated with hydration and adhesiveness of freshwater fish eggs and identify critical molecular mechanisms involved in the adaptive radiation of endemic East Asian cyprinids. We propose that these key egg attributes may function as “magic traits” in this adaptive radiation.

Transfer of the <i>zp3a</i> gene results in changes in egg adhesiveness and buoyancy in transgenic zebrafish

“…The top DEGs included the zp , c-type lectin , and galactosaminyltransferase 1 genes. The zp genes were of particular interest to us, as they have been shown to be significantly up-regulated in the EEs of adhesive eggs compared to semi-buoyant eggs ( Chen et al, 2022 ; Wang et al, 2022 ).…”

“…Mitochondrial DNA-based phylogenetic analysis revealed that the endemic East Asian cyprinid clade evolved diverse spawning habits and reproductive strategies (producing adhesive, demersal, and semi-buoyant eggs) ( Chen et al, 2022 ; Tao et al, 2010 ). Demersal eggs appeared approximately 22 million years ago (Ma), followed by semi-buoyant eggs (18 Ma) and adhesive eggs (14 Ma), coincident with the formation of the Yangtze river-lake system during the mid-Miocene and the burst of diversification of East Asian cyprinids ( Chen et al, 2022 ; Zheng, 2015 ). Studies support the notion that traits related to spawning habits play a major role in shaping biodiversity during adaptive radiation ( Marques et al, 2019 ; McGee et al, 2020 ).…”

“…The acellular egg envelope surrounding teleost oocytes serves multiple functions, including nutrient and water uptake, functional buoyancy, embryo protection, species-specific sperm binding, sperm guidance to the micropyle, and a barrier to contaminants and microorganisms ( Mansour et al, 2009 ). Microstructural analysis using transmission electron microscopy has shown that semi-buoyant eggs, which undergo hydration to adapt to fast-flowing rivers, consist of three-layer structures in the EE, while adhesive eggs, which adhere to water plants in lakes, often consist of four layers ( Bonsignorio et al, 1996 ; Chen et al, 2022 ; Esmaeili &Johal, 2005 ; Rizzo et al, 2002 ; Sano et al, 2017 ). For adhesive eggs, the outer surface of the EE develops an adhesive layer after fertilization or activation ( Murray et al, 2013 ).…”

Alternative signal pathways underly fertilization and egg activation in a fish with contrasting modes of spawning

“…Different modes of spawning in fish have evolved as adaptions to a variety of aquatic habitats. Semi-buoyant eggs represent an adaptation to fast-flowing rivers, while adhesive eggs are associated with lentic environments [ 9 , 24 , 26 ]. The signal pathways underlying fertilization and egg activation of different spawning modes have hitherto been overlooked.…”

“…During early embryogenesis, semi-buoyant eggs rapidly take up water to form a large perivitelline space, which has the function of lifting the egg into suspension in the water column during development. In contrast, adhesive eggs produce an adhesive layer and adhere to aquatic plants or other hard surfaces [ 25 , 26 ]. Despite the clear functional differences in semi-buoyant and adhesive eggs, the different molecular pathways underlying fertilization and egg activation between these spawning modes have yet to be identified.…”