Bi Ke, Bogart James P, Fu Jinzhong
Department of Integrative Biology, University of Guelph, Guelph, Ontario N1G 2W1, Canada.
BMC Evol Biol. 2008 May 22;8:158. doi: 10.1186/1471-2148-8-158.
Unisexual salamanders of the genus Ambystoma exemplify the most ancient lineage of unisexual vertebrates and demonstrate an extremely flexible reproductive system. Unisexual Ambystoma interact with and incorporate genomes from two to four sexual species (A. laterale, A. jeffersonianum,A. texanum, and A. tigrinum), to generate more than 20 genome compositions or biotypes. Unisexual ploidy levels range from diploid to pentaploid, but all contain at least one A. laterale (L) genome. Replacement of nuclear genomes might be responsible for the evolutionary longevity of unisexual Ambystoma but direct evidence for the prevalence of genome replacement in natural populations is absent. Two major puzzling questions have remained unanswered over the last few decades: 1) is genome replacement a common reproductive method in various unisexual populations and, 2) is there an ancient "L" genome that persists in various unisexual genome compositions.
We examined 194 unisexual and 89 A. laterale specimens from 97 localities throughout their range and constructed a genealogy of the "L" genomes using a nuclear DNA marker (L-G1C12) to answer the above questions. Six L-G1C12 haplotypes (A-F) were shared by individuals in various A. laterale and unisexual populations. The general geographical distribution of the haplotypes in unisexual populations conformed to those found in A. laterale, indicating that "L" genomes in unisexuals are obtained from sympatric or nearby populations of A. laterale.
Our data demonstrate that genome replacement frequently occurs in unisexual Ambystoma across their range, and support previous speculations that genome replacement is an important reproductive mechanism that can enhance their evolutionary longevity. Our results show that there is no ancient "L" genome in the unisexual lineages, and no particular "L" genome is favored in any unisexual individual. The presence of an "L" genome in all unisexuals implies that it is important to the maintenance of unisexuals. Nuclear gene genealogy is a powerful tool to examine the historical interaction between sperm-dependent unisexuals and their sexual sperm donors. This methodology could be applicable to many other unisexual lineages to improve our understanding of their reproduction and their ability to persist.
美西螈属的单性蝾螈代表了最古老的单性脊椎动物谱系，并展现出极其灵活的生殖系统。单性美西螈与两到四种有性物种（A. laterale、A. jeffersonianum、A. texanum和A. tigrinum）的基因组相互作用并整合，从而产生20多种基因组组合或生物型。单性蝾螈的倍性水平从二倍体到五倍体不等，但都至少包含一个A. laterale（L）基因组。核基因组的替换可能是单性美西螈进化长寿的原因，但在自然种群中基因组替换普遍存在的直接证据尚不存在。在过去几十年里，有两个主要的令人困惑的问题一直没有得到解答：1）基因组替换在各种单性种群中是否是一种常见的生殖方式，以及2）在各种单性基因组组合中是否存在一个古老的“L”基因组。
我们检查了来自其分布范围内97个地点的194个单性蝾螈标本和89个A. laterale标本，并使用核DNA标记（L-G1C12）构建了“L”基因组的谱系，以回答上述问题。不同的A. laterale和单性种群中的个体共享六种L-G1C12单倍型（A-F）。单性种群中单倍型的总体地理分布与在A. laterale中发现的分布一致，这表明单性蝾螈中的“L”基因组是从同域或附近的A. laterale种群中获得的。
我们的数据表明，基因组替换在单性美西螈的整个分布范围内经常发生，并支持了之前的推测，即基因组替换是一种重要的生殖机制，可以提高它们的进化寿命。我们的结果表明，在单性谱系中不存在古老的“L”基因组，并且在任何单性个体中都没有特定的“L”基因组受到青睐。所有单性蝾螈中都存在“L”基因组，这意味着它对单性蝾螈的维持很重要。核基因谱系是研究依赖精子的单性蝾螈与其有性精子供体之间历史相互作用的有力工具。这种方法可能适用于许多其他单性谱系，以增进我们对它们的繁殖及其生存能力的理解。
