The effect of hybrid transgression on environmental tolerance in experimental yeast crosses.

Evidence is rapidly accumulating that hybridization generates adaptive variation. Transgressive segregation in hybrids could promote the colonization of new environments. Here, we use an assay to select hybrid genotypes that can proliferate in environmental conditions beyond the conditions tolerated by their parents, and we directly compete them against parental genotypes in habitats across environmental clines. We made 45 different hybrid swarms by crossing yeast strains (both Saccharomyces cerevisiae and S. paradoxus) with different genetic and phenotypic divergence. We compared the ability of hybrids and parents to colonize seven types of increasingly extreme environmental clines, representing both natural and novel challenges (mimicking pollution events). We found that a significant majority of hybrids had greater environmental ranges compared to the average of both their parents' ranges (mid-parent transgression), but only a minority of hybrids had ranges exceeding their best parent (best-parent transgression). Transgression was affected by the specific strains involved in the cross and by the test environment. Genetic and phenotypic crossing distance predicted the extent of transgression in only two of the seven environments. We isolated a set of potentially transgressive hybrids selected at the extreme ends of the clines and found that many could directly outcompete their parents across whole clines and were between 1.5- and 3-fold fitter on average. Saccharomyces yeast is a good model for quantitative and replicable experimental speciation studies, which may be useful in a world where hybridization is becoming increasingly common due to the relocation of plants and animals by humans.

Sediment DNA metabarcoding and morphology provide complementary insight into macrofauna and meiobenthos response to environmental gradients in an Arctic glacial fjord.

Arctic fjords ecosystems are highly dynamic, with organisms exposed to various natural stressors along with productivity clines driven by advection of water masses from shelves. The benthic response to these environmental clines has been extensively studied using traditional, morphology-based approaches mostly focusing on macroinvertebrates. In this study we analyse the effects of glacially mediated disturbance on the biodiversity of benthic macrofauna and meiobenthos (meiofauna and Foraminifera) in a Svalbard fjord by comparing morphology and eDNA metabarcoding. Three genetic markers targeting metazoans (COI), meiofauna (18S V1V2) and Foraminifera (18S 37f) were analyzed. Univariate measures of alpha diversity and multivariate compositional dissimilarities were calculated and tested for similarities in response to environmental gradients using correlation analysis. Our study showed different taxonomic composition of morphological and molecular datasets for both macrofauna and meiobenthos. Some taxonomic groups while abundant in metabarcoding data were almost absent in morphology-based inventory and vice versa. In general, species richness and diversity measures in macrofauna morphological data were higher than in metabarcoding, and similar for the meiofauna. Both methodological approaches showed different patterns of response to the glacially mediated disturbance for the macrofauna and the meiobenthos. Macrofauna showed an evident distinction in taxonomic composition and a dramatic cline in alpha diversity indices between the outer and inner parts of fjord, while the meiobenthos showed a gradual change and more subtle responses to environmental changes along the fjord axis. The two methods can be seen as complementing rather than replacing each other. Morphological approach provides more accurate inventory of larger size species and more reliable quantitative data, while metabarcoding allows identification of inconspicuous taxa that are overlooked in morphology-based studies. As different taxa may show different sensitivities to environmental changes, both methods shall be used to monitor marine biodiversity in Arctic ecosystems and its response to dramatically changing environmental conditions.