No abstract

Summary Understanding the molecular basis of how reproductive isolation evolves between individuals from the same species offers valuable insight into patterns of genetic differentiation as well as the onset of speciation [1, 2]. The yeast Saccharomyces cerevisiae constitutes an ideal model partly due to its vast ecological range, high level of genetic diversity [3–6] and laboratory amendable sexual reproduction. Between S. cerevisiae and its sibling species in the Saccharomyces sensu stricto complex, reproductive isolation acts post-zygotically and could be attributed to chromosomal rearrangements [7], cyto-nuclear incompatibility [8, 9] and anti-recombination [10, 11]; although the implication of these mechanisms at the incipient stage of speciation remains unclear due to further divergence in the nascent species. Recently, several studies assessed the onset of intraspecific reproductive isolation in S. cerevisiae by evaluating the effect of the mismatch repair system [12–14] or by fostering incipient speciation using the same initial genetic background [15–18]. Nevertheless, the overall genetic diversity within this species was largely overlooked and no systematic evaluation has been performed. Here, we carried out the first species-wide survey for post-zygotic reproductive isolation in S. cerevisiae. We crossed 60 natural isolates sampled from diverse niches with the reference strain S288c, and identified 16 cases of reproductive isolation with reduced offspring viabilities ranging from 44% to 86%. Using different mapping strategies, we identified reciprocal translocations in a large fraction of all isolates surveyed, indicating that large-scale chromosomal rearrangements might play a major role to the onset of reproductive isolation in this species.

Willows (Salix) and poplars (Populus) are dioecious plants in Salicaceae family. Sex chromosome in poplar genome was consistently reported to be associated with chromosome XIX. In contrast to poplar, this study revealed that chromosome XV was sex chromosome in willow. Previous studies revealed that both ZZ/ZW and XX/XY sex-determining systems could be present in some species of Populus. In this study, sex of S. suchowensis was found to be determined by the ZW system in which the female was the heterogametic gender. Gene syntenic and collinear comparisons revealed macrosynteny between sex chromosomes and the corresponding autosomes between these two lineages. By contrast, no syntenic segments were found to be shared between poplar's and willow's sex chromosomes. Syntenic analysis also revealed substantial chromosome rearrangements between willow's alternate sex chromatids. Since willow and poplar originate from a common ancestor, we proposed that evolution of autosomes into sex chromosomes in these two lineages occurred after their divergence. Results of this study indicate that sex chromosomes in Salicaceae are still at the early stage of evolutionary divergence. Additionally, this study provided valuable information for better understanding the genetics and evolution of sex chromosome in dioecious plants.

Almost all plants in the genus Populus are dioecious (i.e. trees are either male or female), but it is unknown whether dioecy evolved in a common ancestor or independently in different subgenera. Here, we sequence the small peritelomeric X- and Y-linked regions of P. deltoides chromosome XIX. Two genes are present only in the Y-linked region. One is a duplication of a non-Y-linked, female-specifically expressed response regulator, which produces siRNAs that block this gene’s expression, repressing femaleness. The other is an LTR/Gypsy transposable element family member, which generates long non-coding RNAs. Overexpression of this gene in A. thaliana promotes androecium development. We also find both genes in the sex-determining region of P. simonii, a different poplar subgenus, which suggests that they are both stable components of poplar sex-determining systems. By contrast, only the duplicated response regulator gene is present in the sex-linked regions of P. davidiana and P. tremula. Therefore, findings in our study suggest dioecy may have evolved independently in different poplar subgenera.