Gene flow across a hybrid zone maintained by a weak heterogametic incompatibility and positive selection of incompatible alleles

Hybridization between incipient species is more likely to produce sterile or inviable F₁ offspring in the heterogametic (XY or ZW) sex than in the homogametic (XX or ZZ) sex, a phenomenon known as Haldane's rule. Population dynamics associated with Haldane's rule may play an important role in early speciation of sexually reproducing organisms. The dynamics of the hybrid zone maintained by incomplete hybrid inferiority (sterility/inviability) in the heterogametic sex (a ‘weak’ Haldane's rule) caused by a Bateson–Dobzhansky–Muller incompatibility was modelled. The influences and interplays of the strengths of incompatibility, dispersal, density‐dependent regulation (DDR) and local adaptation of incompatible alleles in a scenario of short‐range dispersal (the stepping‐stone model) were examined. It was found that a partial heterogametic hybrid incompatibility could efficiently impede gene flow and maintain characteristic clinal noncoincidence and discordance of alleles. Density‐dependent regulation appears to be an important factor affecting hybrid zone dynamics: it can effectively skew the effects of the partial incompatibility and dispersal as measured by effective dispersal, clinal structures and density depression. Unexpectedly, local adaptation of incompatible alleles in the parental populations, which would be critical for the establishment of the incompatibility, exerts little effect on hybrid zone dynamics. These results strongly support the plausibility of the adaptive origin of hybrid incompatibility and ecological speciation: an adaptive mutation, if it confers a marginal fitness advantage in the local population and happens to cause epistatic inferiority in hybrids, could efficiently drive further genetic divergence that may result in the gene becoming an evolutionary hotspot.     

Misregulation of spermatogenesis genes in Drosophila hybrids is lineage‐specific and driven by the combined effects of sterility and fast male regulatory divergence

Hybrid male sterility is a common outcome of crosses between different species. Gene expression studies have found that a number of spermatogenesis genes are differentially expressed in sterile hybrid males, compared with parental species. Late‐stage sperm development genes are particularly likely to be misexpressed, with fewer early‐stage genes affected. Thus, a link has been posited between misexpression and sterility. A more recent alternative explanation for hybrid gene misexpression has been that it is independent of sterility and driven by divergent evolution of male‐specific regulatory elements between species (faster male hypothesis). The faster male hypothesis predicts that misregulation of spermatogenesis genes should be independent of sterility and approximately the same in both hybrids, whereas sterility should only affect gene expression in sterile hybrids. To test the faster male hypothesis vs. the effect of sterility on gene misexpression, we analyse spermatogenesis gene expression in different species pairs of the Drosophila phylogeny, where hybrid male sterility occurs in only one direction of the interspecies cross (i.e. unidirectional sterility). We find significant differences among genes in misexpression with effects that are lineage‐specific and caused by sterility or fast male regulatory divergence.     

Developmental regulation of the Drosophila tropomyosin I (TmI) gene is controlled by a muscle activator enhancer region that contains multiple cis-elements and binding sites for multiple proteins

Developmental gene regulation in vertebrate somatic muscles involves the cooperative interaction of MEF2 (myocyte-specific enhancer-binding factor 2) and members of the b-HLH (basic helix-loop-helix) family of myogenic factors. Until recently, however, nothing was known about the factors that control the developmental regulation of muscle genes during embryogenesis in Drosophila. The Drosophila Tropomyosin I (TmI) gene contains a proximal and distal muscle enhancer within the first intron that regulates its expression in embryonic/larval and adult muscles. We have recently shown that the 355-bp proximal enhancer contains a binding site for the Drosophila homologue of vertebrate MEF2 and that MEF2 acts cooperatively with a basal level muscle activator region to direct high level muscle expression in transgenic flies. The 92-bp muscle activator region, however, does not contain any consensus E-box (CANNTG) binding site sequences for b-HLH myogenic factors, suggesting the MEF2 may interact with other factors to regulate muscle genes in Drosophila. In this study we have used mutation analysis and germ-line transformation to analyze the cis-acting elements within the muscle activator region that regulate its expression in transgenic flies. We have identified a 71-bp region that is sufficient for low basal level temporal- and muscle-specific expression in the embryo, larva, and adult. Substitution mutations within the muscle activator region have identified several cis-element regions spanning 60-bp that are required for either full or partial muscle activator function. An analysis of proteins that bind to this region by gel mobility shift assay and copper nuclease footprinting has allowed us to identify the sites in this region at which multiple proteins complex and interact. We propose that these cis-elements and the proteins that they bind regulate muscle activator function and together with MEF2 are capable of regulating high level muscle expression. Dev. Genet. 20:297–306, 1997. © 1997 Wiley-Liss, Inc.     

A rapid PCR-aided selection of a rice line containing the Rf-1 gene which is involved in restoration of the cytoplasmic male sterility

Cytoplasmic male sterility (CMS) is widely known in higher plants, the mechanism of which is believed to involve incompatibility between nuclei and cytoplasms. In rice lines with the CMS trait, fertility is restored by the aid of a nuclear-encoded gene, Rf-1, whose locus has been determined in chromosome 10. We found a particular PCR-amplified fragment, designated fL601, that specifically amplified using the DNAs from Rf-1 lines tested as templates. RFLP mapping of the fL601 locus revealed that there are two loci for the fL601, and that both are tightly linked to the Rf-1 locus. Progeny analysis also showed high frequency of their co-segregation. Southern analysis of the genomic DNA demonstrated that the Rf-1 lines shared a unique sequence in the fL601 region. These results enabled us to construct a system for specific detection of the corresponding regions. Utilizing this detection system, we established a simple PCR-mediated selection method for the Rf-1 lines, which may facilitate the breeding for hybrid rice.     

Niche divergence of two closely related Carbula species (Insecta: Hemiptera: Pentatomidae) despite the presence of a hybrid zone

1. The extent to which ecologically divergent selection acts to maintain species boundaries in the presence of hybridisation and gene flow is not well understood. Two parapatric taxa, Carbula humerigera (Uhler) and Carbula putoni (Jakovlev), were used to test the extent to which niche differentiation might sustain divergence of related taxa despite ongoing gene flow. 2. Mitochondrial [cytochrome c oxidase subunit I (COI), cytochrome c oxidase subunit II (COII) and cytochrome b (Cytb)] and nuclear [elongation factor 1‐alpha (EF‐1α)] markers were sequenced from 383 individuals. Clusters of morphological variations were estimated and visualised using principal component analysis (PCA). Haplotype networks were constructed using median‐joining and neighbour‐net algorithm methods. Gene flows were estimated using migrate‐n analyses. Suitable habitats for each species and their sympatric distribution were predicted with ecological niche modelling (ENM). Niche comparisons were conducted using Schoener's D and Warren's I. In addition, PCA, multivariate analysis of variance (manova ) and discriminant function analysis were used to test ecological differentiation. 3. Morphological clusters and network analysis indicated that samples were generally divided into three groups (C. humerigera, C. putoni and hybrids). Ongoing gene flow was detected among the three groups, with abundant magnitudes in the sympatric region. Niche comparison and statistical analysis showed ecological differentiation between C. humerigera and C. putoni. Two potential hybrid zones were predicted in the ENM reconstruction, located along the Yanshan‐Taihang‐Qinling and Taishan mountains. 4. These results reveal a geographically delineated hybrid zone between C. humerigera and C. putoni. These two closely related Carbula species still live in different ecological niches despite hybridisation and ongoing gene flow.     

Characterization of nuclear and cytoplasmic information in the progeny of a somatic hybrid between male sterile Nicotiana tabacum and N. glutinosa

Summary Several nuclear and cytoplasmic characters of the back-crossed progeny of a somatic hybrid between male sterile Nicotiana tabacum (N. debneyi cytoplasm) and N. glutinosa have been analysed. Progeny were obtained by repeated back-crossing of a somatic hybrid with pollen from either N. tabacum or N. glutinosa. Nuclear ribosomal RNA genes (rDNA) were found to be a reliable marker to determine the constitution of nuclear genomes in the progeny. The progeny obtained by back-crossing with N. tabacum pollen maintained uniformity in leaf morphology. On the other hand, variation in leaf morphology was observed in the second back-cross population obtained with N. glutinosa pollen. This may be due to a variable contribution of N. tabacum chromosomes. Segregation of rDNA was also found in individuals of the same back-crossed progeny, but was not related to the chromosome number. The stable inheritance of chloroplast DNA in the back-crossed generation was confirmed regardless of the type of pollen donor. Male sterility was consistently maintained throughout several generations, suggesting that the nuclear genome of either N. tabacum or N. glutinosa does not influence the expression of cytoplasmic male sterility.