Department of Integrative Biology, University of Guelph, Guelph, ON, N1G 2W1, Canada.
Chromosome Res. 2010 Apr;18(3):371-82. doi: 10.1007/s10577-010-9121-3. Epub 2010 Apr 1.
The meiotic mechanism of unisexual salamanders in the genus Ambystoma was previously explained by observing lampbrush chromosomes (LBCs). In polyploid unisexual females, a pre-meiotic endomitotic event doubles the chromosome number so that, after meiotic reduction, the mature eggs have the same ploidy as the female. It was assumed that synapses during meiotic I prophase, which result in observed bivalents, join duplicated sister chromosomes. Previous studies also found LBC quadrivalents in some oocytes that could be explained by occasional synapses between homologs. The discovery of widespread intergenomic exchanges among unisexual populations has prompted new speculations on this meiotic mechanism. Synapses that involve homeologous chromosomes may be frequent during meiosis and could be responsible for intergenomic exchanges and the high embryonic mortality of unisexuals. Furthermore, LBC quadrivalents may be established by associations between homeologous rather than homologous chromosomes. The present study investigated these two important aspects pertaining to the mechanism of intergenomic exchanges: the frequency of homeologous synapses and the relationship between homeologous associations and meiotic quadrivalents. We applied genomic in situ hybridization (GISH) on LBCs from oocytes of 14 triploid and two tetraploid unisexual females. Homeologous bivalents were not observed, and all 13 LBC quadrivalents that we found were the result of homologous synapses and were not associated with any homeologous or exchanged LBCs. Intergenomic exchanges were used as markers to compare the same chromosomes at meiotic diplotene and mitotic metaphase stages. We conclude that contemporary intergenomic exchanges are very rare, and no direct link exists between intergenomic exchanges and high embryonic mortality. The actual mechanisms and evolutionary implications of intergenomic exchanges appear to be complicated and difficult to assess. The application of GISH-type molecular cytogenetic techniques will help to improve our understanding of the role that intergenomic interactions play in the persistence of unisexual Ambystoma and other unisexual vertebrates.
先前，通过观察灯刷染色体（LBC）来解释单性蝾螈属（Ambystoma）的减数分裂机制。在多倍体的单性雌性中，有丝分裂前期的预减数内复制事件使染色体数目加倍，从而使减数分裂后成熟的卵子与雌性具有相同的倍性。人们假设，减数分裂 I 前期的联会导致观察到的二价体，连接复制的姐妹染色体。之前的研究还在一些卵母细胞中发现了 LBC 四价体，可以通过同源染色体之间偶尔的联会来解释。单性群体中广泛存在的基因组间交换的发现促使人们对这种减数分裂机制提出了新的推测。在减数分裂过程中，涉及同源染色体的联会可能很频繁，可能是基因组间交换和单性动物高胚胎死亡率的原因。此外，LBC 四价体可能是由同源染色体之间的联会而不是同源染色体建立的。本研究调查了与基因组间交换机制有关的两个重要方面：同源染色体联会的频率以及同源染色体之间的关联与减数四价体之间的关系。我们应用基因组原位杂交（GISH）对来自 14 个三倍体和两个四倍体单性雌性的卵母细胞的 LBC 进行了研究。未观察到同源二价体，我们发现的所有 13 个 LBC 四价体都是同源联会的结果，与任何同源或交换的 LBC 无关。基因组间交换被用作标记，以比较减数二分体和有丝分裂中期的相同染色体。我们得出的结论是，当代基因组间交换非常罕见，基因组间交换与高胚胎死亡率之间没有直接联系。基因组间交换的实际机制和进化意义似乎很复杂，难以评估。基因组间相互作用在单性蝾螈和其他单性脊椎动物的持续存在中的作用的理解，应用 GISH 型分子细胞遗传学技术将有所帮助。
