• Rama S. Singh

      Articles written in Journal of Genetics

    • Contrasting patterns of genetic structure and evolutionary history as revealed by rnitochondrial DNA and nuclear gene-enzyme variation betweenDrosophila melanogaster andDrosophila simulans

      Lawrence R. Hale Rama S. Singh

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      Drosophila melanogaster and its sibling speciesD. simulans have a cosmopolitan distribution. Studies on nuclear gene-enzyme variation from natural populations of these species reveal that the two have almost equal overall heterozygosity, yetD. simulans populations are significantly less differentiated. However, it is not clear whether this difference in population structure represents a difference in the genetic strategy with which they respond to the same adaptive challenges, or is the result of difference in species history. To help answer this question, we have undertaken an intensive survey of restriction fragment length polymorphisms of mitochondrial DNA (mtDNA) fromD. simulans; the results are compared with those fromD. melanogaster. We surveyed 69 isofemale lines ofD. simulans from four continents and seven lines from the Seychelles Islands. Ten restriction enzymes detected 104 restriction sites in the continental mtDNAs, of which only threeHinf1 sites were variable and account for fourHlnf1 (restriction variants) haplotypes. These four variants were all found in geographically distant locations. By contrast, twenty-three haplotypes were observed inD. melanogaster, many of which were observed in only one population. It would seem, therefore, that these two species have had different histories. Specifically, cosmopolitan populations ofD: simulans are probably products of a comparatively recent expansion from a source population in Africa. These results do not negate differences in their genetic strategy of adaptation, but they do show the importance of historical contingency in the present-day pattern of geographic variation.

    • The molecular basis of speciation: from patterns to processes, rules to mechanisms

      Rob J. Kulathinal Rama S. Singh

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      The empirical study of speciation has brought us closer to unlocking the origins of life’s vast diversity. By examining recently formed species, a number of general patterns, or rules, become apparent. Among fixed differences between species, sexual genes and traits are one of the most rapidly evolving and novel functional classes, and premating isolation often develops earlier than postmating isolation. Among interspecific hybrids, sterility evolves faster than inviability, the X-chromosome has a greater effect on incompatibilities than autosomes, and hybrid dysfunction affects the heterogametic sex more frequently than the homogametic sex (Haldane’s rule). Haldane’s rule, in particular, has played a major role in reviving interest in the genetics of speciation. However, the large genetic and reproductive differences between taxa and the multi-factorial nature of each rule have made it difficult to ascribe general mechanisms. Here, we review the extensive progress made since Darwin on understanding the origin of species. We revisit the rules of speciation, regarding them as landmarks as species evolve through time. We contrast these ‘rules’ of speciation to ‘mechanisms’ of speciation representing primary causal factors ranging across various levels of organization—from genic to chromosomal to organismal. To explain the rules, we propose a new ‘hierarchical faster-sex’ theory: the rapid evolution of sex and reproduction-related (SRR) genes (faster-SRR evolution), in combination with the preferential involvement of the X-chromosome (hemizygous X-effects) and sexually selected male traits (faster-male evolution). This unified theory explains a comprehensive set of speciation rules at both the prezyotic and postzygotic levels and also serves as a cohesive alternative to dominance, composite, and recent genomic conflict interpretations of Haldane’s rule.

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