Grishanin Andrey K, Rasch Ellen M, Dodson Stanley I, Wyngaard Grace A
Department of Biology, James Madison University, Harrisonburg, Virginia 22807, USA.
Evolution. 2006 Feb;60(2):247-56.
Collectively, populations of Acanthocyclops vernalis, a species complex of freshwater copepods, are remarkably similar as to morphology and DNA content, despite variability in chromosome number. Reproductive isolation had been reported among some populations, but with each new investigation the species boundaries and factors that may influence them appeared less clear. To clarify the pattern of biological species within this group of populations, we adopted a comprehensive approach and examined patterns of reproductive isolation in populations for which morphology, chromosome number, DNA content, and 18S rDNA sequences are known. In this study we established nine isofemale lines from four sites in Wisconsin and performed 266 crosses. Crosses within and among these lines were used to relate the degree of reproductive isolation to chromosome differences and to construct a model to explain the origin and maintenance of chromosome number variability. Different gametic and somatic chromosome numbers were observed among specimens within some isofemale lines. In a few cases, gametes with different haploid numbers were produced by a single female. Matings within isofemale lines always produced at least some reproductively successful replicate crosses (produced viable, fertile offspring). Crosses between lines from the same site showed reduced success relative to within-line crosses. Crosses between populations from distant sites showed limited genetic compatibility, producing viable, fertile F1 offspring but infertile F2 adults. One cross between lines with different chromosome numbers (one with 2n = 8 and one with 2n = 10) produced fertile viable offspring, which reproduced for at least 60 generations. These hybrids had either eight or nine chromosomes in the third generation of inbreeding, and eight chromosomes after 20 generations. These hybrids also had reduced nuclear DNA contents at the third generation, a level that persisted through the 20th generation. Successful backcrosses between some hybrids and their parental lines further demonstrated the potential for genetic compatibility among forms with different chromosome numbers. We propose a model in which alterations due to Robertsonian fusions, translocations, and/or loss of chromosomal fragments generate heritable variation, only some of which leads to reproductive isolation. Hence, some of the criteria traditionally used to recognize species boundaries in animals (morphology, DNA content, chromosome number) may not apply to this species complex.
总的来说，春季棘剑水蚤（一种淡水桡足类物种复合体）的种群在形态和DNA含量方面非常相似，尽管染色体数目存在变异。已有报道称一些种群之间存在生殖隔离，但随着每次新的调查，物种界限以及可能影响它们的因素似乎变得越来越不清晰。为了阐明这组种群中的生物物种模式，我们采用了一种综合方法，并研究了已知形态、染色体数目、DNA含量和18S rDNA序列的种群中的生殖隔离模式。在本研究中，我们从威斯康星州的四个地点建立了九条单雌系，并进行了266次杂交。这些系内和系间的杂交用于将生殖隔离程度与染色体差异联系起来，并构建一个模型来解释染色体数目变异的起源和维持。在一些单雌系的标本中观察到了不同的配子染色体数和体细胞染色体数。在少数情况下，单个雌体产生了具有不同单倍体数的配子。单雌系内的交配总是至少产生一些生殖成功的重复杂交（产生了可存活、可育的后代）。来自同一地点的系间杂交相对于系内杂交显示出较低的成功率。来自遥远地点的种群间杂交显示出有限的遗传相容性，产生了可存活、可育的F1后代，但F2代成虫不育。一次不同染色体数目的系间杂交（一个2n = 8，一个2n = 10）产生了可育的可存活后代，这些后代繁殖了至少60代。这些杂种在近交第三代时具有八条或九条染色体，在20代后具有八条染色体。这些杂种在第三代时核DNA含量也降低了，这一水平一直持续到第20代。一些杂种与其亲本系之间成功的回交进一步证明了不同染色体数目的形态之间遗传相容性的潜力。我们提出了一个模型：由于罗伯逊融合、易位和/或染色体片段丢失导致的改变产生了可遗传变异，但只有其中一些导致了生殖隔离。因此，一些传统上用于识别动物物种界限的标准（形态、DNA含量、染色体数目）可能不适用于这个物种复合体。
