School of Biological, Biomedical and Environmental Sciences, University of Hull, Hull, HU6 7RX, UK.
BMC Evol Biol. 2013 Feb 5;13:30. doi: 10.1186/1471-2148-13-30.
Sexual system is a key factor affecting the genetic diversity, population structure, genome structure and the evolutionary potential of species. The sexual system androdioecy - where males and hermaphrodites coexist in populations - is extremely rare, yet is found in three crustacean groups, barnacles, a genus of clam shrimps Eulimnadia, and in the order Notostraca, the tadpole shrimps. In the ancient crustacean order Notostraca, high morphological conservatism contrasts with a wide diversity of sexual systems, including androdioecy. An understanding of the evolution of sexual systems in this group has been hampered by poor phylogenetic resolution and confounded by the widespread occurrence of cryptic species. Here we use a multigene supermatrix for 30 taxa to produce a comprehensive phylogenetic reconstruction of Notostraca. Based on this phylogenetic reconstruction we use character mapping techniques to investigate the evolution of sexual systems. We also tested the hypothesis that reproductive assurance has driven the evolution of androdioecy in Notostraca.
Character mapping analysis showed that sexual system is an extremely flexible trait within Notostraca, with repeated shifts between gonochorism and androdioecy, the latter having evolved a minimum of five times. In agreement with the reproductive assurance hypothesis androdioecious notostracans are found at significantly higher latitudes than gonochoric ones indicating that post glacial re-colonisation may have selected for the higher colonisation ability conferred by androdioecy.
In contrast to their conserved morphology, sexual system in Notostraca is highly labile and the rare reproductive mode androdioecy has evolved repeatedly within the order. Furthermore, we conclude that this lability of sexual system has been maintained for at least 250 million years and may have contributed to the long term evolutionary persistence of Notostraca. Our results further our understanding of the evolution of androdioecy and indicate that reproductive assurance is a recurrent theme involved in the evolution of this sexual system.
性系统是影响物种遗传多样性、种群结构、基因组结构和进化潜力的关键因素。性系统雌雄同体 - 雄性和雌雄同体共存于种群中 - 极为罕见，但在三个甲壳动物群体、藤壶、豆形肢目（Eulimnadia）的一种蛤蜊虾和枝角目（Notostraca）、尾状幼体虾中发现。在古老的甲壳动物枝角目目中，高形态保守性与广泛的性系统多样性形成鲜明对比，包括雌雄同体。对该组性系统进化的理解受到系统发育分辨率差和广泛存在的隐种的影响。在这里，我们使用 30 个分类单元的多基因超级矩阵来产生枝角目目全面的系统发育重建。基于这个系统发育重建，我们使用特征映射技术来研究性系统的进化。我们还测试了生殖保证推动了枝角目雌雄同体进化的假设。
特征映射分析表明，性系统是枝角目中一个极其灵活的特征，雌雄同体和雌雄同体之间存在反复转变，后者至少进化了五次。与生殖保证假说一致，雌雄同体的枝角目目在纬度上明显高于雌雄同体，表明冰川后再殖民可能选择了雌雄同体赋予的更高的殖民能力。
与保守的形态相反，枝角目目中的性系统高度不稳定，罕见的生殖模式雌雄同体在该目中反复进化。此外，我们得出结论，这种性系统的不稳定性至少已经维持了 2.5 亿年，并且可能有助于枝角目的长期进化生存。我们的研究结果进一步了解了雌雄同体的进化，并表明生殖保证是涉及这种性系统进化的一个反复出现的主题。
