Sterility and Gene Expression in Hybrid Males of Xenopus laevis and X. muelleri

BACKGROUND: Reproductive isolation is a defining characteristic of populations that represent unique biological species, yet we know very little about the gene expression basis for reproductive isolation. The advent of powerful molecular biology tools provides the ability to identify genes involved in reproductive isolation and focuses attention on the molecular mechanisms that separate biological species. Herein we quantify the sterility pattern of hybrid males in African Clawed Frogs (Xenopus) and apply microarray analysis of the expression pattern found in testes to identify genes that are misexpressed in hybrid males relative to their two parental species (Xenopus laevis and X. muelleri). METHODOLOGY/PRINCIPAL FINDINGS: Phenotypic characteristics of spermatogenesis in sterile male hybrids (X. laevis x X. muelleri) were examined using a novel sperm assay that allowed quantification of live, dead, and undifferentiated sperm cells, the number of motile vs. immotile sperm, and sperm morphology. Hybrids exhibited a dramatically lower abundance of mature sperm relative to the parental species. Hybrid spermatozoa were larger in size and accompanied by numerous undifferentiated sperm cells. Microarray analysis of gene expression in testes was combined with a correction for sequence divergence derived from genomic hybridizations to identify candidate genes involved in the sterility phenotype. Analysis of the transcriptome revealed a striking asymmetric pattern of misexpression. There were only about 140 genes misexpressed in hybrids compared to X. laevis but nearly 4,000 genes misexpressed in hybrids compared to X. muelleri. CONCLUSIONS/SIGNIFICANCE: Our results provide an important correlation between phenotypic characteristics of sperm and gene expression in sterile hybrid males. The broad pattern of gene misexpression suggests intriguing mechanisms creating the dominance pattern of the X. laevis genome in hybrids. These findings significantly contribute to growing evidence for allelic dominance in hybrids and have implications for the mechanism of species differentiation at the transcriptome level

Genetic and Ecotypic Differentiation in a Californian Plant Polyploid Complex (Grindelia, Asteraceae)

Studies of ecotypic differentiation in the California Floristic Province have contributed greatly to plant evolutionary biology since the pioneering work of Clausen, Keck, and Hiesey. The extent of gene flow and genetic differentiation across interfertile ecotypes that span major habitats in the California Floristic Province is understudied, however, and is important for understanding the prospects for local adaptation to evolve or persist in the face of potential gene flow across populations in different ecological settings. We used microsatellite data to examine local differentiation in one of these lineages, the Pacific Coast polyploid complex of the plant genus Grindelia (Asteraceae). We examined 439 individuals in 10 different populations. The plants grouped broadly into a coastal and an inland set of populations. The coastal group contained plants from salt marshes and coastal bluffs, as well as a population growing in a serpentine grassland close to the coast, while the inland group contained grassland plants. No evidence for hybridization was found at the single location where adjacent populations of the two groups were sampled. In addition to differentiation along ecotypic lines, there was also a strong signal of local differentiation, with the plants grouping strongly by population. The strength of local differentiation is consistent with the extensive morphological variation observed across populations and the history of taxonomic confusion in the group. The Pacific Clade of Grindelia and other young Californian plant groups warrant additional analysis of evolutionary divergence along the steep coast-to-inland climatic gradient, which has been associated with local adaptation and ecotype formation since the classic studies of Clausen, Keck, and Hiesey

Evading the non-continuity equation in the f(R, T) cosmology

Abstract We present a new approach for the f(R, T) gravity formalism, by thoroughly exploring the extra terms of its effective energy-momentum tensor $$T_{\mu \nu }^\mathrm{eff}$$ Tμνeff , which we name $$\tilde{T}_{\mu \nu }$$ T~μν , so that $$T_{\mu \nu }^\mathrm{eff}=T_{\mu \nu }+\tilde{T}_{\mu \nu }$$ Tμνeff=Tμν+T~μν , with $$T_{\mu \nu }$$ Tμν being the usual energy-momentum tensor of matter. Purely from the Bianchi identities, we obtain the conservation of both parts of the effective energy-momentum tensor, rather than the non-conservation of $$T_{\mu \nu }$$ Tμν , originally occurring in the f(R, T) theories. In this way, the intriguing scenario of matter creation, which still lacks observational evidence, is evaded. One is left, then, with two sets of cosmological equations to be solved: the Friedmann-like equations along with the conservation of $$T_{\mu \nu }$$ Tμν and along with the conservation of $$\tilde{T}_{\mu \nu }$$ T~μν . We present a physical interpretation for the conservation of $$\tilde{T}_{\mu \nu }$$ T~μν , which can be related to the presence of stiff matter in the universe. The cosmological consequences of this approach are presented and discussed as well as the benefits of evading the matter energy-momentum tensor non-conservation