Department of Ecology and Evolution, University of Lausanne, Lausanne, Switzerland.
Dipartimento di Scienze Biologiche, Geologiche e Ambientali, University of Bologna, Bologna, Italy.
Evolution. 2024 May 29;78(6):1109-1120. doi: 10.1093/evolut/qpae045.
Reproduction is a key feature of all organisms, yet the way in which it is achieved varies greatly across the tree of life. One striking example of this variation is the stick insect genus Bacillus, in which five different reproductive modes have been described: sex, facultative and obligate parthenogenesis, and two highly unusual reproductive modes: hybridogenesis and androgenesis. Under hybridogenesis, the entire genome from the paternal species is eliminated and replaced each generation by mating with the corresponding species. Under androgenesis, an egg is fertilized, but the developing diploid offspring bear two paternal genomes and no maternal genome, as a consequence of unknown mechanisms. Here, we reevaluate the previous descriptions of Bacillus lineages and the proposed F1 hybrid ancestries of the hybridogenetic and obligately parthenogenetic lineages (based on allozymes and karyotypes) from Sicily, where all these reproductive modes are found. We generate a chromosome-level genome assembly for a facultative parthenogenetic species (B. rossius) and combine extensive field sampling with RADseq and mtDNA data. We identify and genetically corroborate all previously described species and confirm the ancestry of hybrid lineages. All hybrid lineages have fully retained their F1 hybrid constitution throughout the genome, indicating that the elimination of the paternal genome in hybridogens is always complete and that obligate parthenogenesis in Bacillus hybrid species is not associated with an erosion of heterozygosity as known in other hybrid asexuals. Our results provide a stepping stone toward understanding the transitions between reproductive modes and the proximate mechanisms of genome elimination.
繁殖是所有生物的关键特征，但在生命之树上，其实现方式有很大的差异。这种变异的一个显著例子是杆状昆虫属 Bacillus，其中已经描述了五种不同的繁殖方式：性、兼性和强制性孤雌生殖，以及两种非常不寻常的繁殖方式：杂种发生和雄核发生。在杂种发生中，父本物种的整个基因组被消除，并且每一代通过与相应物种交配来取代。在雄核发生中，卵子受精，但由于未知的机制，发育的二倍体后代携带两个父本基因组而没有母本基因组。在这里，我们重新评估了之前对 Bacillus 谱系的描述，以及西西里岛（所有这些繁殖方式都存在）中提出的杂种发生和强制性孤雌生殖谱系的 F1 杂种祖先（基于同工酶和核型）。我们为一个兼性孤雌生殖物种（B. rossius）生成了一个染色体水平的基因组组装，并结合了广泛的实地采样、RADseq 和 mtDNA 数据。我们鉴定并遗传证实了所有以前描述的物种，并确认了杂种谱系的祖先。所有杂种谱系在整个基因组中都完全保留了它们的 F1 杂种组成，这表明杂种发生中的父本基因组的消除总是完全的，并且 Bacillus 杂种物种中的强制性孤雌生殖与其他杂种无性生物中已知的杂合性侵蚀无关。我们的结果为理解繁殖方式之间的转变和基因组消除的近因机制提供了一个垫脚石。
