Department of Plant Pathology, Washington State University, Pullman, USA.
Phytopathology. 2013 Jul;103(7):741-9. doi: 10.1094/PHYTO-08-12-0211-R.
Most Alternaria spp. are considered asexual but recent molecular evolution analyses of Alternaria mating-type genes show that the mating locus is under strong purifying selection, indicating a possible role in sexual reproduction. The objective of this study was to determine the mode of reproduction of an Alternaria alternata sensu lato population causing citrus brown spot in central Florida. Mating type of each isolate was determined, and isolates were sequenced at six putatively unlinked loci. Three genetically distinct subpopulations (SH1, SH4A, and SH4B) were identified using network and Bayesian population structure analyses. Results demonstrate that most subpopulations of A. alternata associated with citrus are clonal but some have the ability to extensively recombine through a cryptic sexual cycle or parasexual cycle. Although isolates were sampled in close physical proximity (≈2,500-m² area), we were able to reject a random mating model using multilocus gametic disequilibrium tests for two subpopulations, SH1 and SH4B, suggesting that these subpopulations were predominantly asexual. However, three recombination events were identified in SH1 and SH4B and localized to individuals of opposite mating type, possibly indicating meiotic recombination. In contrast, in the third subpopulation (SH4A), where only one mating type was present, extensive reticulation was evident in network analyses, and multilocus gametic disequilibrium tests were consistent with recombination. Recombination among isolates of the same mating type suggests that a nonmeiotic mechanism of recombination such as the parasexual cycle may be operating in this subpopulation. The level of gene flow detected among subpopulations does not appear to be sufficient to prevent differentiation, and perhaps future speciation, of these A. alternata subpopulations.
大多数链格孢属（Alternaria spp.）被认为是无性繁殖的，但最近对链格孢交配型基因的分子进化分析表明，交配位点受到强烈的净化选择，这表明其在有性生殖中可能发挥作用。本研究的目的是确定引起佛罗里达州中部柑橘褐斑病的广义链格孢（Alternaria alternata sensu lato）种群的繁殖方式。测定了每个分离株的交配型，并对 6 个假定不连锁的基因座进行了测序。使用网络和贝叶斯种群结构分析确定了三个遗传上不同的亚种群（SH1、SH4A 和 SH4B）。结果表明，与柑橘相关的大多数链格孢亚种群是无性繁殖的，但有些亚种群通过隐性有性循环或准性循环具有广泛重组的能力。尽管分离株是在近距离（≈2500 平方米的区域）采样的，但我们能够通过对两个亚种群（SH1 和 SH4B）的多点配子不平衡测试来拒绝随机交配模型，这表明这些亚种群主要是无性繁殖的。然而，在 SH1 和 SH4B 中发现了三个重组事件，并定位于相反交配型的个体，可能表明减数分裂重组。相比之下，在第三个亚种群（SH4A）中，只有一种交配型存在，网络分析表明存在广泛的网状结构，多点配子不平衡测试与重组一致。相同交配型的分离株之间的重组表明，非减数分裂的重组机制，如准性循环，可能在这个亚种群中起作用。检测到的亚种群间基因流水平似乎不足以阻止这些链格孢亚种群的分化，或许未来会导致物种形成。
