Differences in (G+C) content between species: a commentary on Forsdyke's "chromosomal viewpoint" of speciation

Speciation, natural selection, and networks: three historians versus theoretical population geneticists

In 1913, the geneticist William Bateson called for a halt in studies of genetic phenomena until evolutionary fundamentals had been sufficiently addressed at the molecular level. Nevertheless, in the 1960s, the theoretical population geneticists celebrated a "modern synthesis" of the teachings of Mendel and Darwin, with an exclusive role for natural selection in speciation. This was supported, albeit with minor reservations, by historians Mark Adams and William Provine, who taught it to generations of students. In subsequent decades, doubts were raised by molecular biologists and, despite the deep influence of various mentors, Adams and Provine noted serious anomalies and began to question traditional "just-so-stories." They were joined in challenging the genetic orthodoxy by a scientist-historian, Donald Forsdyke, who suggested that a "collective variation" postulated by Darwin's young research associate, George Romanes, and a mysterious "residue" postulated by Bateson, might relate to differences in short runs of DNA bases (oligonucleotides). The dispute between a small network of historians and a large network of geneticists can be understood in the context of national politics. Contrasts are drawn between democracies, where capturing the narrative makes reversal difficult, and dictatorships, where overthrow of a supportive dictator can result in rapid reversal.

When acting as a reproductive barrier for sympatric speciation, hybrid sterility can only be primary

Animal gametes unite to form a zygote that develops into an adult with gonads that, in turn, produce gametes. Interruption of this germinal cycle by prezygotic or postzygotic reproductive barriers can result in two cycles, each with the potential to evolve into a new species. When the speciation process is complete, members of each species are fully reproductively isolated from those of the other. During speciation a primary barrier may be supported and eventually superceded by a later-appearing secondary barrier. For those holding certain cases of prezygotic isolation to be primary (e.g. elephant cannot copulate with mouse), the onus is to show that they had not been preceded over evolutionary time by periods of postzygotic hybrid inviability (genically determined) or sterility (genically or chromosomally determined). Likewise, the onus is upon those holding cases of hybrid inviability to be primary (e.g. Dobzhansky–Muller epistatic incompatibilities) to show that they had not been preceded by periods, however brief, of hybrid sterility. The latter, when acting as a sympatric barrier causing reproductive isolation, can only be primary. In many cases, hybrid sterility may result from incompatibilities between parental chromosomes that attempt to pair during meiosis in the gonad of their offspring (Winge-Crowther-Bateson incompatibilities). While such incompatibilities have long been observed on a microscopic scale, there is growing evidence for a role of dispersed finer DNA sequence differences (i.e. in base k-mers).

Spore-autonomous fluorescent protein expression identifies meiotic chromosome mis-segregation as the principal cause of hybrid sterility in yeast

Genome-wide sequence divergence between populations can cause hybrid sterility through the action of the anti-recombination system, which rejects crossover repair of double strand breaks between nonidentical sequences. Because crossovers are necessary to ensure proper segregation of homologous chromosomes during meiosis, the reduced recombination rate in hybrids can result in high levels of nondisjunction and therefore low gamete viability. Hybrid sterility in interspecific crosses of Saccharomyces yeasts is known to be associated with such segregation errors, but estimates of the importance of nondisjunction to postzygotic reproductive isolation have been hampered by difficulties in accurately measuring nondisjunction frequencies. Here, we use spore-autonomous fluorescent protein expression to quantify nondisjunction in both interspecific and intraspecific yeast hybrids. We show that segregation is near random in interspecific hybrids. The observed rates of nondisjunction can explain most of the sterility observed in interspecific hybrids through the failure of gametes to inherit at least one copy of each chromosome. Partially impairing the anti-recombination system by preventing expression of the RecQ helicase SGS1 during meiosis cuts nondisjunction frequencies in half. We further show that chromosome loss through nondisjunction can explain nearly all of the sterility observed in hybrids formed between two populations of a single species. The rate of meiotic nondisjunction of each homologous pair was negatively correlated with chromosome size in these intraspecific hybrids. Our results demonstrate that sequence divergence is not only associated with the sterility of hybrids formed between distantly related species but may also be a direct cause of reproductive isolation in incipient species.

William Bateson, Richard Goldschmidt, and Non-Genic Modes of Speciation

Sometimes a cross between two individuals that appear to belong to the same species produces a sterile offspring (i.e., their hybrid is sterile). Thus, the two individuals appear reproductively isolated from each other. If each could find a compatible mate, then new species might emerge. At issue is whether the form of hybrid sterility that precedes sympatric differentiation into species is, in the general case, of genic or non-genic origin. Several recent papers lend the authority of William Bateson to the genic hypothesis, referring to the "Bateson–Dobzhansky–Muller hypothesis". All these papers cite a 1996 paper that, in turn, cites a 1909 paper of Bateson. However, from 1902 until 1926 the latter espoused a non-genic hypothesis that today would be classified as "chromosomal". Analysis of Bateson's 1909 text reveals no recantation. Bateson's non-genic view was similar to that advanced by Richard Goldschmidt in the 1940s. However, Bateson proposed a contribution from parents of abstract factors that, together in their hybrids, complement to bring about a negative effect (hybrid sterility). In contrast, Goldschmidt proposed that normally parents contribute complementary factors making parental chromosomes compatible at meiosis in their hybrids, which hence are fertile (i.e., the parental factors work together to produce a positive effect). When the factors are not sufficiently complementary the parental chromosomes are incompatible in their hybrids, which hence are sterile. The non-genic Batesonian–Goldschmidtian abstractions are now being fleshed-out chemically in terms of DNA base-composition differences.

Molecular sex: The importance of base composition rather than homology when nucleic acids hybridize

On learning that nucleic acid hybridization had been achieved in a test tube, Huxley hailed the discovery of "molecular sex." The description was apt, since sex involves recombination, which requires hybridization that, in turn, depends on a successful homology search. Conversely, when the homology search fails, recombination fails. In yeast, this failure has been attributed to "simple sequence divergence." But sequence divergence does not impair nucleic acid hybridization simply. Most natural single-stranded nucleic acids are predisposed to adopt higher-order structures containing stem-loops. Tomizawa showed that the rate-limiting step in the hybridization of single-stranded sequences is an initial "kissing" exploration between complementary loops, which must first be appropriately extruded and aligned. Successful duplex formation requires successful synchronization of matching higher-ordered structures, which depends, not so much on the degree of similarity between their base sequences as on the closeness of their base compositions (GC%). In these terms, we can understand how the anti-recombinational effect of GC% differences supports the duplication both of genes within a genome and of genomes within a genus (speciation).

Chromosomal speciation: a reply

The "genic" and the "non-genic" (chromosomal) hypotheses for the predominant mechanism by which species diverge into two have long been in contention. In 1998 Coyne and Orr attacked certain formulations of the chromosomal hypothesis on the grounds that they required macromutations (structural changes in chromosomes). In 1999 I replied that numerous independent micromutations (single DNA base changes) should suffice (GC% hypothesis). Kliman et al., with the support of Coyne and Charlesworth, have presented various counterarguments, to which the present paper responds with evidence that GC% differences are primary to genic differences and would operate by changing the structure of stem-loops extruded from duplex DNAs. Chromosomes attempting to align by means of complementary loop-loop interactions would fail if GC% differences exceeded a critical threshold. This would disrupt meiosis (hybrid sterility) and the parents of organisms with failed meiosis would be reproductively isolated from each other. If they could find new mates with which they were GC-compatible, then new species could emerge. The model leads to predictions consistent with several lines of evidence. The GC% version of the chromosomal hypothesis has a sound basis and deserves at least as much attention as its genic rival.

Chromosomal inversions and the reproductive isolation of species

Recent genetic studies have suggested that many genes contribute to differences between closely related species that prevent gene exchange, particularly hybrid male sterility and female species preferences. We have examined the genetic basis of hybrid sterility and female species preferences in Drosophila pseudoobscura and Drosophila persimilis, two occasionally hybridizing North American species. Contrary to findings in other species groups, very few regions of the genome were associated with these characters, and these regions are associated also with fixed arrangement differences (inversions) between these species. From our results, we propose a preliminary genic model whereby inversions may contribute to the speciation process, thereby explaining the abundance of arrangement differences between closely related species that co-occur geographically. We suggest that inversions create linkage groups that cause sterility to persist between hybridizing taxa. The maintenance of this sterility allows the species to persist in the face of gene flow longer than without such inversions, and natural selection will have a greater opportunity to decrease the frequency of interspecies matings.