Research Department for Limnology, University of Innsbruck, Mondsee, Austria.
The New Zealand Institute for Plant and Food Research Ltd., Nelson, New Zealand.
Sci Rep. 2022 Dec 3;12(1):20900. doi: 10.1038/s41598-022-25566-8.
Intraspecific genome size (GS) variation in Eukaryotes is often mediated by additional, nonessential genomic elements. Physically, such additional elements may be represented by supernumerary (B-)chromosomes or by large heterozygous insertions into the regular chromosome set. Here we analyze meiotic transmission patterns of Megabase-sized, independently segregating genomic elements (ISEs) in Brachionus asplanchnoidis, a planktonic rotifer that displays an up to two-fold intraspecific GS variation due to variation in size and number of these elements. To gain insights into the meiotic transmission patterns of ISEs, we measured GS distributions of haploid males produced by individual mother clones using flow cytometry and compared these distributions to theoretical distributions expected under a range of scenarios. These scenarios considered transmission biases resembling (meiotic) drive, or cosegregation biases, e.g., if pairs of ISEs preferentially migrated towards the same pole during meiosis. We found that the inferred transmission patterns were diverse and ranged from positive biases (suggesting drive) to negative biases (suggesting drag), depending on rotifer clone and its ISE composition. Additionally, we obtained evidence for a negative cosegregation bias in some of the rotifer clones, i.e., pairs of ISEs exhibited an increased probability of migrating towards opposite poles during meiosis. Strikingly, these transmission and segregation patterns were more similar among members of a genetically homogeneous inbred line than among outbred members of the population. Comparisons between early and late stages of haploid male embryonic development (e.g., young synchronized male eggs vs. hatched males) showed very similar GS distributions, suggesting that transmission biases occur very early in male development, or even during meiosis. Very large genome size was associated with reduced male embryonic survival, suggesting that excessive amounts of ISEs might be detrimental to male fitness. Altogether, our results indicate considerable functional diversity of ISEs in B. asplanchnoidis, with consequences on meiotic transmission and embryonic survival.
真核生物种内基因组大小（GS）的变异通常是由额外的非必需基因组元件介导的。从物理上讲，这些额外的元件可以由超数（B）染色体或大量异源插入到正常染色体组中表现出来。在这里，我们分析了多碱基大小的、独立分离的基因组元件（ISEs）在浮游轮虫 Brachionus asplanchnoidis 中的减数分裂传递模式，由于这些元件的大小和数量的变化，B. asplanchnoidis 表现出两倍的种内 GS 变化。为了深入了解 ISEs 的减数分裂传递模式，我们使用流式细胞术测量了来自单个母本克隆的单倍体雄性的 GS 分布，并将这些分布与一系列场景下预期的理论分布进行了比较。这些场景考虑了类似于（减数分裂）驱动或共分离偏倚的传递偏倚，例如，在减数分裂过程中，一对 ISE 是优先向同一极迁移还是向相反极迁移。我们发现，推断的传递模式是多样的，从正偏倚（表明驱动）到负偏倚（表明拖曳）不等，这取决于轮虫克隆及其 ISE 组成。此外，我们在一些轮虫克隆中获得了负共分离偏倚的证据，即 ISE 对在减数分裂过程中向相反极迁移的概率增加。引人注目的是，这些传递和分离模式在遗传同质的近交系成员之间比在种群的杂交成员之间更为相似。早期和晚期单倍体雄性胚胎发育阶段（例如，年轻的同步雄性卵与孵化的雄性）之间的比较显示出非常相似的 GS 分布，这表明在雄性发育的早期，甚至在减数分裂过程中就存在传递偏倚。非常大的基因组大小与雄性胚胎存活率降低有关，这表明过多的 ISEs 可能对雄性适应度有害。总的来说，我们的结果表明，B. asplanchnoidis 中的 ISEs 具有相当大的功能多样性，对减数分裂传递和胚胎存活率有影响。
