Data from: Spontaneous hybrids between native and exotic Rubus in the Western United States produce offspring both by apomixis and by sexual recombination

Facultative asexual reproduction is a trait commonly found in invasive species. With a combination of sexual and asexual reproductive modes, such species may adapt to new environments via sexual recombination during range expansion, while at the same time having the benefits of asexuality such as the maintenance of fitness effects that depend upon heterozygosity. In the Western United States, native species of Rubus (Rosaceae) reproduce sexually whereas exotic naturalized Rubus species reproduce by pseudogamous apomixis. We hypothesized that new asexual lineages of Rubus could arise from hybridization in this range. To detect hybridization between native and exotic Rubus, we genotyped 579 individuals collected across California, Oregon, and Washington with eight nuclear microsatellites and two chloroplast markers. Principal Coordinate Analysis and Bayesian clustering revealed a limited amount of hybridization of the native R. ursinus with the exotic R. armeniacus and R. pensilvanicus as well as cultivated varieties. Genetic distances between these hybrids and their offspring indicated that both R. ursinus x armeniacus and R. ursinus x pensilvanicus produced a mix of apomictic and sexual seeds, with sexual seeds being more viable. Although neither of these hybrid types is currently considered invasive, they model the early stages of evolution of new invasive lineages, given the potential for fixed heterosis and the generation of novel genotypes. The hybrids also retain the ability to increase their fitness via sexual recombination and natural selection. Mixed reproductive systems such as those described here may be an important step in the evolution of asexual invasive species.兼性无性繁殖（Facultative asexual reproduction）是入侵物种（invasive species）中常见的性状。这类物种兼具有性与无性生殖模式，既可在种群扩张（range expansion）过程中通过有性重组（sexual recombination）适应新生境，同时还能享有无性生殖的优势，例如维持依赖杂合性（heterozygosity）的适合度效应（fitness effects）。在美国西部，本土悬钩子属（Rubus）植物隶属于蔷薇科（Rosaceae），其行有性生殖；而外来归化的悬钩子属物种则通过假配无融合生殖（pseudogamous apomixis）进行繁殖。我们提出假说：该区域内的悬钩子属新无性谱系（lineages）可能由杂交（hybridization）事件产生。为检测本土与外来悬钩子属之间的杂交情况，我们对采集自加利福尼亚州、俄勒冈州与华盛顿州的579个个体，采用8个核微卫星（nuclear microsatellites）标记与2个叶绿体标记（chloroplast markers）进行了基因分型。主坐标分析（Principal Coordinate Analysis）与贝叶斯聚类（Bayesian clustering）结果显示，本土的R. ursinus与外来的R. armeniacus、R. pensilvanicus以及栽培品种之间仅存在有限的杂交现象。对这些杂交种与其后代的遗传距离分析表明，R. ursinus × armeniacus与R. ursinus × pensilvanicus这两类杂交组合均可产生无融合生殖与有性两类种子，且有性种子的活力（viable）更高。尽管目前这两类杂交组合均未被认定为入侵物种，但鉴于其具备固定杂种优势（fixed heterosis）并可产生全新基因型的潜力，它们可为新型入侵谱系的早期演化过程提供研究模型。此类杂交种还保留了通过有性重组与自然选择（natural selection）提升自身适合度的能力。诸如本文所描述的混合生殖系统（mixed reproductive systems），可能在无性入侵物种的演化进程中扮演关键的过渡步骤。