From egg-laying to live-bearing: Unravelling the genetics of a major evolutionary transition

点击翻译按钮获取中文摘要

The process of reproduction is incredibly complex. From attracting the right mate, being genetically compatibility, and incubating a baby, there are many steps that must all work in concert. Each step is under stark natural selection because if unsuccessful then an individual's genes are not passed to the next generation. Tinkering with this process sees like tempting a Darwinian dead-end. So how can major, transformative changes in reproductive process happen during evolution? The evolution of live-bearing (viviparity) from egg-laying (oviparity) in animals is an example of such a transformation, which has profound and wide-ranging consequences for a species. Yet live-bearing has evolved many times independently, in fishes, amphibians, early in the origin of therian mammals, and more than 100 times in reptiles. Usually there is no opportunity to watch how changes from egg-laying to live-bearing happen because they occurred deep in the evolutionary past. Lizards are are an exception as some species have evolved live-bearing quite recently. Most species are completely either egg-laying or live-bearing: eggs are incubated for a relatively short time and then laid with a thick, calcified shell (oviparity) or babies are nurtured in the uterus until fully developed and then born covered with only a thin membrane (viviparity). The genetic basis of live-bearing is not known in any vertebrate. The recent revolution in genomic sequencing technologies is allowing evolutionary biologists to address questions never before possible. One of these is the extent to which similar, complex adaptations have evolved from the same genetic bases across lineages. This issue is pivotal to understand how, and at what rate, natural selection shapes genomes so animals can adapt to their environment. Despite the 300 million years of evolutionary distance between reptiles and mammals, the basic structures, physiology, and molecular mechanisms of pregnancy are comparable between lizards and mammals, so the answer is relevant beyond just reptiles. The first step is to identify the genetic basis of live-bearing in a single species.Europe's common lizard (Zootoca vivipara) has the remarkable attribute of both reproductive modes within one species. The reproductive modes are genetically fixed and egg-layer and live-bearers are found in different geographic areas, except in one part of the Alps where oviparous and viviparous individuals come into contact and interbreed. This hybrid zone offers a unique and amazing 'natural experiment' where the genetics of reproductive traits can be studied. This project will be the first to identify the genetic mechanisms of reproductive mode To do so we will focus on the common lizard. Our objectives are to: (1) Characterise the genetic basis of reproductive mode and its traits shown by mothers, such as eggshell structure and the developmental stage at which embryos are laid as eggs or born as fully developed neonates. This is done by an experiment in the hybrid zone; (2) Quantify how oviparous individuals' and viviparous individuals' genomes mix through hybridization, and locate the genetic variations that are under strong natural selection in either reproductive mode; (3) Resolve the phylogenetic tree of the entire species complex and determine the timing and order of changes in reproductive style. Surprisingly, there is some evidence that reversals back to oviparity might occur in common lizards, but the evidence has not been well supported. Our genomic experiments will identify if multiple independent origins of viviparity, or a reversal to oviparity, have occurred in the species' history. We will identify whether those transitions involve the same genetic mechanism each time.This research will examine for the first time the genetic architecture of reproductive mode, with direct relevance to biodiversity adaptation and reproductive biology.