Data from: Hybridization between two cryptic filamentous brown seaweeds along the shore: analysing pre- and post-zygotic barriers in populations of individuals with varying ploidy levels

We aimed to study the importance of hybridization between two cryptic species of the genus Ectocarpus, a group of filamentous algae with haploid-diploid life cycles that include the principal genetic model organism for the brown algae. In haploid-diploid species, the genetic structure of the two phases of the life cycle can be analysed separately in natural populations. Such life cycles provide a unique opportunity to estimate the frequency of hybrid genotypes in diploid sporophytes and meiotic recombinant genotypes in haploid gametophytes allowing the effects of reproductive barriers preventing fertilization or preventing meiosis to be untangle. The level of hybridization between E. siliculosus and E. crouaniorum was quantified along the European coast. Clonal cultures (568 diploid, 336 haploid) isolated from field samples were genotyped using cytoplasmic and nuclear markers to estimate the frequency of hybrid genotypes in diploids and recombinant haploids. We identified admixed individuals using microsatellite loci, classical assignment methods and a newly developed Bayesian method (XPloidAssignment), which allows the analysis of populations that exhibit variations in ploidy level. Over all populations, the level of hybridization was estimated at 8.7%. Hybrids were exclusively observed in sympatric populations. More than 98% of hybrids were diploids (40% of which showed signs of aneuploidy) with a high frequency of rare alleles. The near absence of haploid recombinant hybrids demonstrates that the reproductive barriers are mostly post-zygotic and suggests that abnormal chromosome segregation during meiosis following hybridisation of species with different genome sizes could be a major cause of interspecific incompatibility in this system.

本研究旨在探究褐藻纲核心遗传模式生物所属的外囊藻属（Ectocarpus）两个隐存种之间的杂交重要性。外囊藻属为一类具有单倍-二倍体生活史的丝状藻类。在单倍-二倍体物种中，可分别对自然种群中生活史两个阶段的遗传结构进行分析。此类生活史为研究提供了独特契机：可分别估算二倍体孢子体中的杂交基因型频率，以及单倍体配子体中的减数分裂重组基因型频率，从而厘清阻碍受精或减数分裂的生殖隔离所产生的效应。
本研究沿欧洲海岸量化了硅质外囊藻（Ectocarpus siliculosus）与克劳氏外囊藻（Ectocarpus crouaniorum）之间的杂交水平。研究人员对野外样本分离得到的克隆培养物（568份二倍体、336份单倍体）采用细胞质与细胞核标记进行基因分型，以估算二倍体中的杂交基因型频率与重组单倍体频率。本研究借助微卫星位点、经典归属分析法以及新开发的贝叶斯方法XPloidAssignment，对存在倍性水平变异的种群进行分析，以此鉴定遗传混合个体。
所有种群的整体杂交水平估算为8.7%。杂交个体仅见于同域种群中。超过98%的杂交个体为二倍体（其中40%表现出非整倍性特征），且这类二倍体杂种携带高频的稀有等位基因。几乎不存在单倍体重组杂种这一结果表明，此类生殖隔离以合子后隔离为主；同时也提示，基因组大小存在差异的物种杂交后，减数分裂过程中出现的染色体分离异常，可能是该系统中种间不亲和性的主要诱因。