Hintermann & Weber SA, Avenue des Alpes 25, 1820, Montreux, Switzerland.
Department of Ecology and Evolution, University of Lausanne, Biophore Building, 1015, Lausanne, Switzerland.
BMC Evol Biol. 2019 Aug 5;19(1):164. doi: 10.1186/s12862-019-1482-4.
Hybridogenesis can represent the first stage towards hybrid speciation where the hybrid taxon eventually weans off its parental species. In hybridogenetic water frogs, the hybrid Pelophylax kl. esculentus (genomes RL) usually eliminates one genome from its germline and relies on its parental species P. lessonae (genomes LL) or P. ridibundus (genomes RR) to perpetuate in so-called L-E and R-E systems. But not exclusively: some all-hybrid populations (E-E system) bypass the need for their parental species and fulfill their sexual cycle via triploid hybrid frogs. Genetic surveys are essential to understand the great diversity of these hybridogenetic dynamics and their evolution. Here we conducted such study using RAD-sequencing on Pelophylax from southern Switzerland (Ticino), a geographically-isolated region featuring different assemblages of parental P. lessonae and hybrid P. kl. esculentus.
We found two types of hybridogenetic systems in Ticino: an L-E system in northern populations and a presumably all-hybrid E-E system in the closely-related southern populations, where P. lessonae was not detected. In the latter, we did not find evidence for triploid individuals from the population genomic data, but identified a few P. ridibundus (RR) as offspring from interhybrid crosses (LR × LR).
Assuming P. lessonae is truly absent from southern Ticino, the putative maintenance of all-hybrid populations without triploid individuals would require an unusual lability of genome elimination, namely that P. kl. esculentus from both sexes are capable of producing gametes with either L or R genomes. This could be achieved by the co-existence of L- and R- eliminating lineages or by "hybrid amphigamy", i. e. males and females producing sperm and eggs among which both genomes are represented. These hypotheses imply that polyploidy is not the exclusive evolutionary pathway for hybrids to become reproductively independent, and challenge the classical view that hybridogenetic taxa are necessarily sexual parasites.
杂种发生可代表杂种形成的第一阶段，在此阶段杂种后代最终摆脱其亲本物种。在杂种基因渐渗的泽蛙中，杂种 Pelophylax kl. esculentus（基因组 RL）通常会从其生殖系中消除一个基因组，并依赖其亲本物种 P. lessonae（基因组 LL）或 P. ridibundus（基因组 RR）在所谓的 L-E 和 R-E 系统中延续。但并非排他性的：一些全杂种群体（E-E 系统）绕过了对其亲本物种的需求，并通过三倍体杂种蛙来完成其有性循环。遗传调查对于理解这些杂种发生动态及其进化的多样性至关重要。在这里，我们使用 RAD 测序对来自瑞士南部提契诺州（Ticino）的 Pelophylax 进行了这样的研究，该地区是一个地理隔离的地区，具有不同的亲本 P. lessonae 和杂种 P. kl. esculentus 集合。
我们在提契诺州发现了两种杂种发生系统：北部种群的 L-E 系统和南部种群中可能全是杂种的 E-E 系统，在后者中没有检测到 P. lessonae。在后者中，我们没有从群体基因组数据中找到三倍体个体的证据，但从杂交种间杂交（LR × LR）中鉴定出少数 P. ridibundus（RR）后代。
假设 P. lessonae 确实不存在于提契诺州南部，那么没有三倍体个体的全杂种种群的维持将需要基因组消除的异常不稳定性，即雌雄两性均能够产生具有 L 或 R 基因组的配子。这可以通过 L- 和 R- 消除谱系的共存或“杂种两性生殖”来实现，即雄性和雌性产生的精子和卵子中都包含两个基因组。这些假设意味着多倍体不是杂种变得独立繁殖的唯一进化途径，并挑战了杂种发生类群必然是性寄生虫的经典观点。
