Meiotic transmission patterns of additional genomic elements in Brachionus asplanchnoidis, a rotifer with intraspecific genome size variation.

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.