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.本研究旨在探究水云属（Ectocarpus）两个隐存物种间的杂交重要性。水云属为一类具备单倍-二倍体生活史的丝状藻类，其中包含褐藻的核心模式遗传物种。对于具有单倍-二倍体生活史的物种而言，可在自然种群中分别解析其生活史两个阶段的遗传结构。此类生活史为相关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，借此厘清阻止受精或抑制减数分裂的生殖障碍所产生的效应。本研究对欧洲沿岸的硅化水云（E. siliculosus）与克朗水云（E. crouaniorum）间的杂交水平进行了量化分析。研究人员从野外样本中分离得到克隆培养物（含568份二倍体样本、336份单倍体样本），并利用细胞质与细胞核分子标记对其进行基因分型，以估算二倍体群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、经典聚类分配方法，以及新开发的贝叶斯分析工具（XPloidAssignment）对携带混合遗传背景的个体进行鉴定；该工具可用于分析存在倍性水平变异的种群。在所有被检测种群中，杂交发生率估算为8.7%。且杂交个体仅在同域分布种群中被发现。超过98%的杂交个体为二倍体，其中40%表现出非整倍性特征，且这类二倍体杂交个体携带高频稀有等位基因。几乎未检测到单倍体重组杂交个体，这表明该类生殖障碍以合子后障碍为主；同时也提示，在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生殖不亲和性的主要诱因。