Do genome size differences within Brachionus asplanchnoidis (Rotifera, Monogononta) cause reproductive barriers among geographic populations?

Abstract

Genome size in the rotifer Brachionus asplanchnoidis, which belongs to the B. plicatilis species complex, is greatly enlarged and extremely variable . Such variation raises the question whether large genome size differences among individuals might cause reproductive barriers, which could trigger speciation within this group by restricting gene flow across populations. To test this hypothesis, we used B. asplanchnoidis clones from three geographic populations and conducted assays to quantify reproductive isolation among clones differing in genome size, and we examined the population structure of all three populations using amplified fragment length polymorphisms (AFLPs). AFLPs indicated that these populations were genetically separated, but we also found hints of natural gene flow. Clones from different populations with genome size differences of up to 1.7-fold could interbred successfully in the laboratory and give rise to viable, fertile 'hybrid' offspring. Genome sizes of these 'hybrids' were intermediate between those of their parents, and fitness in terms of male production, population growth, and egg development time was not negatively affected. Thus, we found no evidence for reproductive isolation or nascent speciation within B. asplanchnoidis. Instead, our results suggest that gene flow within this species can occur despite a remarkably large range of genome sizes.

Within-population genome size variation is mediated by multiple genomic elements that segregate independently during meiosis

“…Genome size was highly variable in the two Austrian populations (genome sizes ranging from 414 to 792 Mbp), whereas clones isolated from Lake Nakuru sediments differed little from each other (426 ± 5.1 Mbp, mean and standard deviation). The two clones from the Mongolian site had distinct and relatively large genome sizes of 652 and 732 Mbp (Data from (31)).…”

“…2, 3). Previous studies have documented the species status of B. asplanchnoidis (30, 31), and have shown that the populations from Austria, Mongolia and Lake Nakuru are genetically distinct, but also experience natural gene flow on a larger geographic scale (31). Thus, in the case of B. asplanchnoidis we can exclude any unrecognized cryptic species.…”

“…Clones with divergent genome size can mate with each other and produce viable and fertile offspring, which are intermediate in genome size between their parents but show some variation (Supplementary Fig. 4, see also (31)). Genome size responds to artificial selection with extremely high heritability.…”

“…Resting eggs of rotifers were collected in the field from Obere Halbjochlacke (OHJ, N 47°47‘11“,E 16°50‘31“) and from Runde Lacke (N 47°47‘08“, E 16°47‘34“), two small alkaline playa lakes in Burgenland (Austria) in 2011. Resting eggs from Lake Nakuru (Kenya) and the two Mongolian clones were obtained from colleagues and have been previously described in detail (26, 31). All rotifers were cultured as clones, consisting of the asexual descendants of the female that initially hatched from a resting egg (for details, see Supplementary methods).…”

Preface: evolving rotifers, evolving science

“…This species complex is frequently used in aquaculture, as food for fish, and in ecological and evolutionary studies. As a result, several papers deal with members of this species complex, including the ecological and evolutionary consequences of its taxonomic, systematic, and nomenclatural intricacies (Gabaldón et al, 2017;Michaloudi et al, 2017;Mills et al, 2017;Riss et al, 2017), dormancy , population dynamics (Kamizono et al, 2017), and culture methods .…”