Spolsky Christina, Phillips Christopher A, Uzzell Thomas
Department of Ecology, Ethology and Evolution, University of Illinois, Urbana, IL, 61801, USA.
Evolution. 1992 Dec;46(6):1935-1944. doi: 10.1111/j.1558-5646.1992.tb01179.x.
Ambystoma platineum, a unisexual clonal triploid taxon of mole salamander, originated by hybridization between the Mendelian species A. jeffersonianum and A. laterale. Studies of lampbrush chromosomes indicated that A. platineum reproduces gynogenetically, that is, sperm from a sexual host species is required to activate egg development but makes no genetic contribution to the developing embryo. Nevertheless, electrophoretic diversity in populations of some hybrid Ambystoma suggested continual in situ recreation of unisexual hybrids and bidirectional gene exchange between the parental species and the hybrids. A. platineum usually lives with, and is sexually dependent on, one of its parental species, A. jeffersonianum. In central Indiana, however, A. platineum populations have shifted their host dependency to A. texanum. Such A. texanum-dependent populations of A. platineum provide an almost ideal system for studying reproductive mode in A. platineum, because both replacement of a jeffersonianum or laterale genome of A. platineum by a texanum genome, and movement of genes from A. platineum to the host species, A. texanum, would be readily detected by electrophoretic markers. Our samples of A. texanum provided no evidence for the transfer of jeffersonianum or laterale genes into A. texanum. Similarly, among 32 A. platineum sampled from six localities in east-central Illinois and central Indiana, we find no texanum alleles, and thus no evidence for genome replacement. The one diploid hybrid individual contained only a jeffersonianum and a laterale genome; because of the absence of either parental species from these populations, this hybrid could only have come from a diploid ovum produced by A. platineum. Both morphometric and electrophoretic results for the two tetraploid individuals indicate that they resulted from fertilization of triploid oocytes of A. platineum by sperm of A. texanum. Because genome replacement in A. texanum-dependent populations of A. platineum is irreversible, the persistence of A. platineum in A. texanum-dependent populations demonstrates conclusively that the major mode of reproduction in A. platineum populations is clonal: A. platineum produces mainly triploid eggs that develop gynogenetically.
美西钝口螈（Ambystoma platineum）是一种单性克隆三倍体的鼹钝口螈分类单元，由孟德尔物种杰斐逊钝口螈（A. jeffersonianum）和侧条钝口螈（A. laterale）杂交产生。对灯刷染色体的研究表明，美西钝口螈进行雌核发育繁殖，也就是说，需要有性宿主物种的精子来激活卵子发育，但对发育中的胚胎没有遗传贡献。然而，一些杂交美西钝口螈种群中的电泳多样性表明，单性杂种在原地不断重新产生，并且亲本物种与杂种之间存在双向基因交换。美西钝口螈通常与它的亲本物种之一杰斐逊钝口螈生活在一起，并且在性方面依赖于它。然而，在印第安纳州中部，美西钝口螈种群已经将它们的宿主依赖性转移到了德克萨斯钝口螈（A. texanum）。这种依赖德克萨斯钝口螈的美西钝口螈种群为研究美西钝口螈的繁殖模式提供了一个几乎理想的系统，因为美西钝口螈的杰斐逊钝口螈或侧条钝口螈基因组被德克萨斯钝口螈基因组取代，以及基因从美西钝口螈转移到宿主物种德克萨斯钝口螈，都可以通过电泳标记很容易地检测到。我们的德克萨斯钝口螈样本没有提供杰斐逊钝口螈或侧条钝口螈基因转移到德克萨斯钝口螈的证据。同样，在从伊利诺伊州中东部和印第安纳州中部的六个地点采集的32只美西钝口螈样本中，我们没有发现德克萨斯钝口螈等位基因，因此也没有基因组取代的证据。唯一的二倍体杂种个体只包含一个杰斐逊钝口螈基因组和一个侧条钝口螈基因组；由于这些种群中没有任何一个亲本物种，这个杂种只能来自美西钝口螈产生的二倍体卵子。两个四倍体个体的形态测量和电泳结果都表明，它们是由德克萨斯钝口螈的精子使美西钝口螈的三倍体卵受精产生的。由于依赖德克萨斯钝口螈的美西钝口螈种群中的基因组取代是不可逆的，依赖德克萨斯钝口螈的种群中美西钝口螈的持续存在确凿地证明了美西钝口螈种群的主要繁殖模式是克隆：美西钝口螈主要产生三倍体卵子，通过雌核发育进行发育。
