Divergence of X-linked trans regulatory proteins and the misexpression of gene targets in sterile Drosophila pseudoobscura hybrids

Background The genetic basis of hybrid incompatibilities is characterized by pervasive cases of gene interactions. Sex chromosomes play a major role in speciation and X-linked hybrid male sterility (HMS) genes have been identified. Interestingly, some of these genes code for proteins with DNA binding domains, suggesting a capability to act as trans-regulatory elements and disturb the expression of a large number of gene targets. To understand how interactions between trans- and cis-regulatory elements contribute to speciation, we aimed to map putative X-linked trans-regulatory elements and to identify gene targets with disrupted gene expression in sterile hybrids between the subspecies Drosophila pseudoobscura pseudoobscura and D. p. bogotana. Results We find six putative trans-regulatory proteins within previously mapped X chromosome HMS loci with sequence changes that differentiate the two subspecies. Among them, the previously characterized HMS gene Overdrive (Ovd) had the largest number of amino acid changes between subspecies, with some substitutions localized within the protein’s DNA binding domain. Using an introgression approach, we detected transcriptional responses associated with a sterility/fertility Ovd allele swap. We found a network of 52 targets of Ovd and identified cis-regulatory effects among target genes with disrupted expression in sterile hybrids. However, a combined analysis of polymorphism and divergence in non-coding sequences immediately upstream of target genes found no evidence of changes in candidate regulatory proximal cis-elements. Finally, peptidases were over-represented among target genes. Conclusions We provide evidence of divergence between subspecies within the DNA binding domain of the HMS protein Ovd and identify trans effects on the expression of 52 gene targets. Our results identify a network of trans-cis interactions with possible effects on HMS. This network provides molecular evidence of gene × gene incompatibilities as contributors to hybrid dysfunction.

Aborting meiosis allows recombination in sterile diploid yeast hybrids

Hybrids between diverged lineages contain novel genetic combinations but an impaired meiosis often makes them evolutionary dead ends. Here, we explore to what extent an aborted meiosis followed by a return-to-growth (RTG) promotes recombination across a panel of 20 Saccharomyces cerevisiae and S. paradoxus diploid hybrids with different genomic structures and levels of sterility. Genome analyses of 275 clones reveal that RTG promotes recombination and generates extensive regions of loss-of-heterozygosity in sterile hybrids with either a defective meiosis or a heavily rearranged karyotype, whereas RTG recombination is reduced by high sequence divergence between parental subgenomes. The RTG recombination preferentially arises in regions with low local heterozygosity and near meiotic recombination hotspots. The loss-of-heterozygosity has a profound impact on sexual and asexual fitness, and enables genetic mapping of phenotypic differences in sterile lineages where linkage analysis would fail. We propose that RTG gives sterile yeast hybrids access to a natural route for genome recombination and adaptation.

A Biosystematic Study of Microseris Subgenus Monermos (Compositae: Cichorieae)

<p>The taxonomy of Microseris subgenus Monermos (Hook. f.) Chambers (Compositae) is investigated. Two species are recognized, namely M. scapigera (sol. ex A. Cunn.) Sch.-Bip. and M. lanceolata (Walp.) Sch.-Bip., which are confined to New Zealand and Australia respectively. In M. scapigera, taxonomic subdivision was not practicable, the formae of Allan (1961) not being upheld. Microseris lanceolata is subdivided into three groups which are described informally as races. The races are viewed as probable subspecies but need further study before this status can be confirmed. The two species are described and illustrated and details are given for each on typification, synonymy, chromosome number, distribution, ecology and geographical variation. Strong self-incompatibility is prevalent in both M. scapigera and M. lanceolata. The only exceptions in the populations examined were in M. scapigera, in which two populations were only moderately self incompatible, and another was substantially self-compatible. The last population appeared to have morphological and behavioural adaptations to promote autogamy. Artificial hybridizations were made within and between the two species. Microseris scapigera and M. lanceolata were freely or poorly intercrossable according to the populations used, and formed semi-fertile or sterile hybrids. Semi-fertile hybrids were virtually blocked from forming a F2 generation (fruit set was very low and no fruits germinated) but they cold be backcrossed. Fertility in BC1 hybrids was mostly higher than in the F1, but was not restored to normal. Microseris scapigera and M. lanceolata race 1 appear to have largely homologous chromosomes. Attempts to cross the species of subgenus Monermos with M. borealis (subgenus Apargidium) were unsuccessful</p>

A Biosystematic Study of Microseris Subgenus Monermos (Compositae: Cichorieae)

<p>The taxonomy of Microseris subgenus Monermos (Hook. f.) Chambers (Compositae) is investigated. Two species are recognized, namely M. scapigera (sol. ex A. Cunn.) Sch.-Bip. and M. lanceolata (Walp.) Sch.-Bip., which are confined to New Zealand and Australia respectively. In M. scapigera, taxonomic subdivision was not practicable, the formae of Allan (1961) not being upheld. Microseris lanceolata is subdivided into three groups which are described informally as races. The races are viewed as probable subspecies but need further study before this status can be confirmed. The two species are described and illustrated and details are given for each on typification, synonymy, chromosome number, distribution, ecology and geographical variation. Strong self-incompatibility is prevalent in both M. scapigera and M. lanceolata. The only exceptions in the populations examined were in M. scapigera, in which two populations were only moderately self incompatible, and another was substantially self-compatible. The last population appeared to have morphological and behavioural adaptations to promote autogamy. Artificial hybridizations were made within and between the two species. Microseris scapigera and M. lanceolata were freely or poorly intercrossable according to the populations used, and formed semi-fertile or sterile hybrids. Semi-fertile hybrids were virtually blocked from forming a F2 generation (fruit set was very low and no fruits germinated) but they cold be backcrossed. Fertility in BC1 hybrids was mostly higher than in the F1, but was not restored to normal. Microseris scapigera and M. lanceolata race 1 appear to have largely homologous chromosomes. Attempts to cross the species of subgenus Monermos with M. borealis (subgenus Apargidium) were unsuccessful</p>

DESIGNING SCHEMES FOR SEED PRODUCTION OF SPRING RAPE F1 HYBRIDS BASED ON CMS

As the result of the research conducted in 2018–2020 at the All-Russian Rapeseed Research Institute, a scheme of primary seed farming of self-pollinated lines and F1 hybrids of spring rape has been developed, over 45 varieties have been created, including self-pollinated lines, simple interline and three-line hybrids. A comparative appraisal of spring rape inbred lines (female parents) and hybrids of the two most common systems of cytoplasmic male sterility (CMS) – Polima (LHS-1, LMS-1, LMS-2) and Ogura (LCS-4, LCS-5, LCS-6), was made. Yield traits distinctiveness in simple sterile hybrids of Polima and Ogura types was studied; it has been asserted that when compared to inbred maternal lines the traits predominate in hybrids, the indices are higher by 24.8% in the hybrids based on sterile Ogura cytoplasm and by 10.7% in the hybrids on sterile Polima cytoplasm. In our studies, self-pollinated lines, the maternal components of the hybrids, are inferior to the hybrids in terms of crude fat content in seeds, it is 6.5% lower than in the hybrid forms for the CMS Pol system and 17.3% lower for the forms of the CMS Ogu system. It was determined that the seed yield of CMS systems hybrids of Ogura type is on average 22.0% higher than the seed yield of Polima hybrids. The starting material for primary seed production of rapeseed hybrids based on CMS has been developed.

Aborting meiosis overcomes hybrid sterility

Hybrids between species or diverged lineages contain fundamentally novel genetic combinations but an impaired meiosis often makes them evolutionary dead ends. Here, we explored to what extent and how an aborted meiosis followed by a return-to-growth (RTG) promotes recombination across a panel of 20 yeast diploid backgrounds with different genomic structures and levels of sterility. Genome analyses of 284 clones revealed that RTG promoted recombination and generated extensive regions of loss-of-heterozygosity in sterile hybrids with either a defective meiosis or a heavily rearranged karyotype, whereas RTG recombination was reduced by high sequence divergence between parental subgenomes. The RTG recombination preferentially occurred in regions with local sequence homology and in meiotic recombination hotspots. The loss-of-heterozygosity had a profound impact on sexual and asexual fitness, and enabled genetic mapping of phenotypic differences in sterile lineages where linkage or association analyses failed. We propose that RTG gives sterile hybrids access to a natural route for genome recombination and adaptation.One sentence summaryAborting meiosis followed by a return to mitotic growth promotes evolution by genome wide-recombination in sterile yeast hybrids.

Intra- and Inter-Specific Crosses among Centaurea aspera L. (Asteraceae) Polyploid Relatives—Influences on Distribution and Polyploid Establishment

How polyploids become established is a long-debated question, especially for autopolyploids that seem to have no evolutionary advantage over their progenitors. The Centaurea aspera polyploid complex includes diploid C. aspera and two related tetraploids C. seridis and C. gentilii. Our purpose was to study the mating system among these three taxa and to analyze its influence on polyploid establishment. The distribution and ploidy level of the Moroccan populations, and forced intra- and inter-specific crosses were assessed. Allotetraploid C. seridis produced more cypselae per capitulum in the intra-specific crosses. It is a bigger plant and autogamous, and previous studies indicated that selfing forces the asymmetric formation of sterile hybrids. All these characteristics help C. seridis to avoid the minority-cytotype-exclusion effect and become established. Inter-specific hybridization was possible between C. aspera and C. gentilii, and with the symmetric formation of hybrids. However, 49% of the hybrid cypselae were empty, which probably reveals postzygotic barriers. Autotetraploid C. gentilii produced the same number of cypselae per capitulum as those of the diploid parental, has an indistinguishable field phenotype, is allogamous, and symmetrically produces hybrids. Therefore, C. gentilii does not seem to have the same competitive advantages as those of C. seridis.

Parthenogenesis as a Solution to Hybrid Sterility: The Mechanistic Basis of Meiotic Distortions in Clonal and Sterile Hybrids

Hybrid sterility is a hallmark of speciation, but the underlying molecular mechanisms remain poorly understood. Here, we report that speciation may regularly proceed through a stage at which gene flow is completely interrupted, but hybrid sterility occurs only in male hybrids whereas female hybrids reproduce asexually. We analyzed gametogenic pathways in hybrids between the fish species Cobitis elongatoides and C. taenia, and revealed that male hybrids were sterile owing to extensive asynapsis and crossover reduction among heterospecific chromosomal pairs in their gametes, which was subsequently followed by apoptosis. We found that polyploidization allowed pairing between homologous chromosomes and therefore partially rescued the bivalent formation and crossover rates in triploid hybrid males. However, it was not sufficient to overcome sterility. In contrast, both diploid and triploid hybrid females exhibited premeiotic genome endoreplication, thereby ensuring proper bivalent formation between identical chromosomal copies. This endoreplication ultimately restored female fertility but it simultaneously resulted in the obligate production of clonal gametes, preventing any interspecific gene flow. In conclusion, we demonstrate that the emergence of asexuality can remedy hybrid sterility in a sex-specific manner and contributes to the speciation process.