No evidence for accumulation of deleterious mutations and fitness degradation in clonal fish hybrids: Abandoning sex without regrets

Despite its inherent costs, sexual reproduction is ubiquitous in nature, and the mechanisms to protect it from a competitive displacement by asexuality remain unclear. Popular mutation-based explanations, like the Muller's ratchet and the Kondrashov's hatchet, assume that purifying selection may not halt the accumulation of deleterious mutations in the nonrecombining genomes, ultimately leading to their degeneration. However, empirical evidence is scarce and it remains particularly unclear whether mutational degradation proceeds fast enough to ensure the decay of clonal organisms and to prevent them from outcompeting their sexual counterparts. To test this hypothesis, we jointly analysed the exome sequences and the fitness-related phenotypic traits of the sexually reproducing fish species and their clonal hybrids, whose evolutionary ages ranged from F1 generations to 300 ky. As expected, mutations tended to accumulate in the clonal genomes in a time-dependent manner. However, contrary to the predictions, we found no trend towards increased nonsynonymity of mutations acquired by clones, nor higher radicality of their amino acid substitutions. Moreover, there was no evidence for fitness degeneration in the old clones compared with that in the younger ones. In summary, although an efficacy of purifying selection may still be reduced in the asexual genomes, our data indicate that its efficiency is not drastically decreased. Even the oldest investigated clone was found to be too young to suffer fitness consequences from a mutation accumulation. This suggests that mechanisms other than mutation accumulation may be needed to explain the competitive advantage of sex in the short term.

In the foothill zone-Sabanejewia balcanica (Karaman 1922), in the lowland zone-Sabanejewia bulgarica (Drensky, 1928): Myth or reality?

The status of golden loaches (genus Sabanejewia) in the region of Central Europe and Balkans is still ambiguous. The greatest controversy is caused by species Sabanejewia balcanica and S. bulgarica. Both species are characterized by a wide spectrum of morphological variability and overlapping of distinguishing features, which then lead to difficulties in their determination. Previous phylogenetic studies aimed on the resolving of their taxonomic status did not include samples from their type localities and so led to a lack of their true distribution in this region. Therefore, the main aim of this study was to identify taxonomic status of golden loaches populations in the region of the middle Danube basin and adjacent areas on the model territory of Slovakia. For this purpose, we used novelty approach (morphological, molecular, and microhabitat) and we also included the missing samples from the type localities of both species. Based on mtDNA all the Slovakian samples reflected haplotype richness revealed on the type locality of S. bulgarica, although the genetic distances from other representatives of the genus Sabanejewia occurring are not significant. Within the morphology, we have revealed a great measure of variability in studied populations, which is largely caused by different habitat conditions and thus representing a phenotypic plasticity of these fish.

Parasitic infection of Cobitis elongatoides Băcescu & Mayer, 1969 by zoonotic metacercariae Clinostomum complanatum (Rudolphi, 1814)

An infection of zoonotic Clinostomum complanatum metacercariae with potential human transmission was recorded close to fish farms in the Tisa River Basin of Slovakia and Ukraine. The prevalence varied from 19.4% to 81.3%, and the intensity of infection varied from 7 to 41. The results of a generalized linear model predicted a positive trend for the Cobitis elongatoides host and a standard length and intensity of infection, with females having a higher number of parasites. However, no significant impact was found of the intensity of infection on Clark's condition of the host. The metacercariae were primarily located in the anterior part of the host's body. Our study also showed significant evidence that water velocity affects the number of C. complanatum metacercariae, regardless of the host's (Cobitis) microhabitat.