Integrative Biology of Marine Organisms (BIOM), Sorbonne University, CNRS, Oceanological Observatory of Banyuls, Banyuls-sur-Mer, France.
Department of Plant Biotechnology and Bioinformatics, Ghent University, Belgium.
Genome Biol Evol. 2021 Oct 1;13(10). doi: 10.1093/gbe/evab216.
Although sex is now accepted as a ubiquitous and ancestral feature of eukaryotes, direct observation of sex is still lacking in most unicellular eukaryotic lineages. Evidence of sex is frequently indirect and inferred from the identification of genes involved in meiosis from whole genome data and/or the detection of recombination signatures from genetic diversity in natural populations. In haploid unicellular eukaryotes, sex-related chromosomes are named mating-type (MTs) chromosomes and generally carry large genomic regions where recombination is suppressed. These regions have been characterized in Fungi and Chlorophyta and determine gamete compatibility and fusion. Two candidate MT+ and MT- alleles, spanning 450-650 kb, have recently been described in Ostreococcus tauri, a marine phytoplanktonic alga from the Mamiellophyceae class, an early diverging branch in the green lineage. Here, we investigate the architecture and evolution of these candidate MT+ and MT- alleles. We analyzed the phylogenetic profile and GC content of MT gene families in eight different genomes whose divergence has been previously estimated at up to 640 Myr, and found evidence that the divergence of the two MT alleles predates speciation in the Ostreococcus genus. Phylogenetic profiles of MT trans-specific polymorphisms in gametologs disclosed candidate MTs in two additional species, and possibly a third. These Mamiellales MT candidates are likely to be the oldest mating-type loci described to date, which makes them fascinating models to investigate the evolutionary mechanisms of haploid sex determination in eukaryotes.
尽管性现在被认为是真核生物普遍存在且古老的特征，但在大多数单细胞真核生物谱系中，仍然缺乏对性的直接观察。性的证据通常是间接的，是从整个基因组数据中识别参与减数分裂的基因和/或从自然种群的遗传多样性中检测重组特征推断出来的。在单倍体单细胞真核生物中，与性相关的染色体被命名为交配型（MT）染色体，通常携带大量重组受到抑制的基因组区域。这些区域在真菌和绿藻门中已经得到了描述，决定配子的兼容性和融合。最近在海洋浮游植物甲藻（Ostreococcus tauri）中描述了两个候选的 MT+和 MT-等位基因，跨度为 450-650 kb，甲藻属于绿藻早期分支的 Mamiellophyceae 类。在这里，我们研究了这些候选 MT+和 MT-等位基因的结构和进化。我们分析了八个不同基因组中 MT 基因家族的系统发育谱和 GC 含量，这些基因组的分化此前估计高达 640 Myr，发现了两个 MT 等位基因分化先于牡蛎属物种形成的证据。在配子体同源基因中的 MT 跨种多态性的系统发育谱揭示了另外两个物种的候选 MT，可能还有第三个物种。这些 Mamiellales MT 候选基因可能是迄今为止描述的最古老的交配型基因座，这使它们成为研究真核生物单倍体性别决定进化机制的迷人模型。
