On 2015 Jan 23, Donald Forsdyke commented:

GC% DIFFERENCES NOT CAUSED BY CONVENTIONAL SELECTION

Its average base composition (GC%) is a characteristic of a biological species. The present work questions the view that GC% differences between species reflect responses to conventional selective pressures on organism function. Thus, the result “challenges the causes and possible functional roles (if any) of GC content variations in grass and Monocot genomes” (1). Likewise, in earlier work, the authors noted that “it is not clear why GC content in introns should also be selected for. Thus, we think that selective hypotheses are not clearly established and are currently insufficient to explain all the data adequately” (2).

To resolve this it would be interesting to examine the GC% values of sympatric, so-called “sibling species” (espèces jumelles, Geschwisterarten). Here phenotypic differences are minimal. Indeed, it has been shown that very small differences in GC% should suffice to spark speciation. These initiating GC% differences could later be obscured by pressures on the phenotype that affect GC%. But when such phenotypic differentiation was minimal, traces of these initiating events might remain (3).

Speciation is still mainly studied in complex organisms. Virus species that infect the same host cell have less scope for developing phenotypic differences and can be construed as sibling species. Indeed, we might recall that the mid-20th century revolution in molecular biology owed much to physicists who studied the simplest living systems – viruses that infect bacteria.

Considering the retroviruses HIV1 and HTLV1 that both infect CD4 T lymphocytes, we find that HIV1 has one of the lowest GC% values known and HTLV1 has one of the highest GC% values known. Large differences are also found when other related viral species share a host cell. Since minute GC% differences can initiate divergence into recombinationally isolated species, it can be assumed that, in the absence of major superceding phenotypic differences, these GC% differences have remained and expanded in HIV1 and HTLV1 (4).

Viewed from a selective perspective, an ancestral retrovirus by virtue of avoiding recombinational blending with its cell-mates, should have been able to develop sufficient functional variation “in sympatry” to achieve full speciation while retaining phenotypic characters needed for the shared intracellular environment. All this harkens back to Romanes who in 1886 proposed that initiation of divergence into species could precede subsequent phenotypic changes (5).

(1) Clément Y, Fustier M-A, Nabholz B, Glémin S. (2015) Genome Biology and Evolution. (In press) doi:10.1093/gbe/evu278

(2) Glémin S, Clément Y, David J, Ressayre A. (2014) GC content evolution in coding regions of angiosperm genomes: a unifying hypothesis. Trends in Genetics 30, 263-270.

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

(4) Forsdyke DR (2014) Implications of HIV RNA structure for recombination, speciation, and the neutralism-selectionism controversy. Microbes and Infection 16, 96-103 doi: 10.1016/j.micinf.2013.10.017.

(5) Romanes GJ (1886) Physiological selection: An additional suggestion on the origin of species. Journal of the Linnaean Society, Zoology 19, 337-411.

On 2015 Jan 23, Donald Forsdyke commented:

GC% DIFFERENCES NOT CAUSED BY CONVENTIONAL SELECTION

Its average base composition (GC%) is a characteristic of a biological species. The present work questions the view that GC% differences between species reflect responses to conventional selective pressures on organism function. Thus, the result “challenges the causes and possible functional roles (if any) of GC content variations in grass and Monocot genomes” (1). Likewise, in earlier work, the authors noted that “it is not clear why GC content in introns should also be selected for. Thus, we think that selective hypotheses are not clearly established and are currently insufficient to explain all the data adequately” (2).

To resolve this it would be interesting to examine the GC% values of sympatric, so-called “sibling species” (espèces jumelles, Geschwisterarten). Here phenotypic differences are minimal. Indeed, it has been shown that very small differences in GC% should suffice to spark speciation. These initiating GC% differences could later be obscured by pressures on the phenotype that affect GC%. But when such phenotypic differentiation was minimal, traces of these initiating events might remain (3).

Speciation is still mainly studied in complex organisms. Virus species that infect the same host cell have less scope for developing phenotypic differences and can be construed as sibling species. Indeed, we might recall that the mid-20th century revolution in molecular biology owed much to physicists who studied the simplest living systems – viruses that infect bacteria.

Considering the retroviruses HIV1 and HTLV1 that both infect CD4 T lymphocytes, we find that HIV1 has one of the lowest GC% values known and HTLV1 has one of the highest GC% values known. Large differences are also found when other related viral species share a host cell. Since minute GC% differences can initiate divergence into recombinationally isolated species, it can be assumed that, in the absence of major superceding phenotypic differences, these GC% differences have remained and expanded in HIV1 and HTLV1 (4).

Viewed from a selective perspective, an ancestral retrovirus by virtue of avoiding recombinational blending with its cell-mates, should have been able to develop sufficient functional variation “in sympatry” to achieve full speciation while retaining phenotypic characters needed for the shared intracellular environment. All this harkens back to Romanes who in 1886 proposed that initiation of divergence into species could precede subsequent phenotypic changes (5).

(1) Clément Y, Fustier M-A, Nabholz B, Glémin S. (2015) Genome Biology and Evolution. (In press) doi:10.1093/gbe/evu278

(2) Glémin S, Clément Y, David J, Ressayre A. (2014) GC content evolution in coding regions of angiosperm genomes: a unifying hypothesis. Trends in Genetics 30, 263-270.

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

(4) Forsdyke DR (2014) Implications of HIV RNA structure for recombination, speciation, and the neutralism-selectionism controversy. Microbes and Infection 16, 96-103 doi: 10.1016/j.micinf.2013.10.017.

(5) Romanes GJ (1886) Physiological selection: An additional suggestion on the origin of species. Journal of the Linnaean Society, Zoology 19, 337-411.