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  1. Feb 2018
    1. On 2016 Jul 03, Donald Forsdyke commented:


      Beginning by citing William Bateson’s magnificent 1909 paper, this elegant study focuses on the role of gene products in the hybrid sterility that can lead to speciation. However, there is an “inherent difficulty of distinguishing when, during the course of speciation, [genic] incompatibilities [resulting in hybrid sterility] may have arisen.” Indeed, the authors’ calculations “imply that most [genic] hybrid incompatibility factors identified in well-established, reproductively isolated species, are likely to have arisen since speciation.”

      Thus there is a distinction between initiation and maintenance. While the genes studied are held to play a role in “contributing to genetic conflicts resulting in reproductive isolation,” this role is one of “contributing to the maintenance of postzygotic reproductive isolation,” not contributing to its initiation. Under the heading “temporal origins of reproductive incompatibility,” it is held “unlikely that either Taf1 or agt was involved in the origin of reproductive isolation between D. simulans and D. mauritiana.”

      On the other hand Bateson had argued that the initial origin of species could often be non-genic (1). There are a large number of sequence differences in the HMS1 region that are non-genic, or intronic, or change codons synonymously (Table 1), and the authors note “considerable opportunity for interactions among sites.” This could affect DNA structure. When such interactions change, the secondary structure of the extruded DNA segments involved in meiotic pairing between D. simulans and D. mauritiana chromosomes could also change.

      The hybrid sterility resulting from such “internal genomic conflicts” could have initiated the divergence, and the observed genic changes would have arisen subsequently. So it can be concluded that “any [genic] incompatibility factor between D. simulans and D. mauritiana is very likely to have evolved since their divergence.” Yet, the authors do not mention the growing evidence for a non-genic basis for initiation of divergence (2-4).

      1.Forsdyke DR (1999) Relationship of Romanes' "intrinsic" variability of the reproductive system, and Bateson's "residue", to the species-dependent component of the base composition, (C+G)%. J Theor Biol 201:47-61 Forsdyke DR, 1999%.")

      2.Forsdyke DR (2001) The Origin of Species Revisited. McGill-Queen’s University Press, Montreal.

      3.Forsdyke DR (2010) George Romanes, William Bateson, and Darwin's "Weak Point." Notes and Records of the Royal Society 64:139-154 (doi:10.1098/rsnr.2009.0045)

      4.Forsdyke DR (2016) Evolutionary Bioinformatics, 3rd edition, (Springer, New York).

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