The Rise of Human Chromosome 2: The Fertility Problem
This essay is the second of a series authored by Dave Wisker, Graduate Student in Molecular Ecology at the University of Central Missouri.
As I wrote in the previous essay in this series, Intelligent Design advocate Casey Luskin doesn’t think the fusion which produced human chromosome 2 could have become fixed in the human population:
Miller may have found good empirical evidence for a chromosomal fusion event. But our experience with mammalian genetics tells us that such a chromosomal aberration could have created a non-viable mutant, or a normal individual who could not produce viable offspring. Thus, Neo-Darwinism has a hard time explaining why such a random fusion event was somehow advantageous.
Luskin (and other ID/creationist apologists I’ve seen on internet discussion fora) maintain that the fusion which resulted in human chromosome 2 must have had drastic negative effects on the fertility of heterozygotes for the fusion. This reduction in fitness, they argue, would effectively prevent the propagation of the fusion throughout the population. On the surface, this sounds like an effective argument, since it is known that translocations and fusions can have such a negative effect by producing non-balanced gametes (see PZ Myers’s article on his blog Pharyngula for a detailed explanation). However, anyone familiar with the cytogenetic literature of mammals knows that one cannot claim that these rearrangements greatly decrease fertility with any certainty, since there are numerous examples where such an expected reduction does not occur.
For example, daMota and da Silva (1998) observed centric fusions in goats that had no discernable effect on fertility:
The results suggest the involvement of chromosomes 6 and 15 in the fusion demonstrated by G-banding in prometaphase cells. The Brazilian sample of animals carrying structural rearrangements did not present any reduction in fertility, suggesting the existence of prezygotic selection against unbalanced gametes
They also cite several other goat studies which found the same thing. In rodents, Nachman and Myers (1989) found a similar situation in a population of marsh rats:
The observation of karyotypic uniformity in most species has led to the widespread belief that selection limits chromosomal change. We report an unprecedented amount of chromosomal variation in a natural population of the South American marsh rat Holochlus brasiliensis. This variation consists of four distinct classes of chromosomal rearrangements: whole-arm translocations, pericentric inversions, variation in the amount of euchromatin, and variation in number and kind of supernumerary (B) chromosomes. Twenty-six karyotypes are present among 42 animals. Observations of the natural population over a 7-year period and breeding experiments with captive animas indicate that heterozygous individuals suffer no detectable reduction in fitness. This is at odds with a central assumption in current models of chromosomal speciation and provides a firm rejection of the view that selection necessarily restricts chromosomal change.
In another study, Bardhan and Sharma (2000) studied heterozygote fertility for numerous translocations and fusions in mice, and concluded:
The three chromosomal species exhibit a high incidence of polymorphisms for Robertsonian fusions and pericentric inversions. Breeding experiments and histological analysis of testis show that heterozygosity for pericentric inversions and Robertsonian fusions had no effect on fertility.
Nachman and Myers (1989) reported similar results in marsh rats:
The observation of karyotypic uniformity in most species has led to the widespread belief that selection limits chromosomal change. We report an unprecedented amount of chromosomal variation in a natural population of the South American marsh rat Holochlus brasiliensis. This variation consists of four distinct classes of chromosomal rearrangements: whole-arm translocations, pericentric inversions,variation in the amount of euchromatin, and variation in number and kind of supernumerary (B) chromosomes. Twenty-six karyotypes are present among 42 animals. Observations of the natural population over a 7-year period and breeding experiments with captive animals indicate that heterozygous individuals suffer no detectable reduction in fitness. This is at odds with a central assumption in current models of chromosomal speciation and provides a firm rejection of the view that selection necessarily restricts chromosomal change.
Researchers in speciation are interested in the fertility effects of chromosomal rearrangements because reduced fertility in heterozygotes can be an effective barrier to gene flow between populations differing by such rearrangements. But they have come to the conclusion that this reduced fertility is not as common as cytogenetic theory predicts. Coyne and Orr (1998) summed up the situation this way:
A further problem with chromosomal speciation is that it depends critically on the semisterility of hybrids who are heterozygous for chromosome rearrangements. It is not widely appreciated, however, that heterozygous rearrangements theoretically expected to be deleterious (e.g. fusions and pericentric inversions) in reality often enjoy normal fitness, probably because segregation is regular or recombination is prevented (see discussion in Coyne et al. (1997)).
Spirito (1998) pointed out that the type of rearrangement was not a reliable indicator of its effect on fertility:
[quote] In conclusion, the reduction in fitness due to the presence of a chromosomal rearrangement (especially in the case of inversions and Robertsonian rearrangements) is not foreseeable a priori solely on the basis of the nature of the structural rearrangement. The absence of definite rules means that it is necessary to experimentally analyze the level of selection against the heterozygote for each particular rearrangement of evolutionary interest. (p.321).
So, it should be clear that Casey Luskin’s remarks are simply not reflective of the current thinking in mammalian cytogenetics. In the next essay, I will take Spirito’s advice and examine the situation in mammals and specifically in humans more closely, so that we can get a better picture of the factors affecting the fixation of Human Chromosome 2.
References:
Bardhan A and T Sharma (2000). Meiosis and speciation: a study in a speciating Mus terricolor complex. J. Genet. 79: 105-111
Coyne, JA and HA Orr (1998).The evolutionary genetics of speciation. Phil. Trans. R. Soc. Lond. B 353: 287-305
da Mota LSLS and RAB da Silva (1998). Centric fusion in goats (Capra hircus): Identification of a 6/15 translocation by high resolution chromosome banding . Genet. Mol. Biol. 21(1): S1415-47571998000100012(online publication)
Nachman MW and P Myers (1989). Exceptional chromosomal mutations in a rodent population are not strongly underdominant. PNAS 86: 6666-6670
Spirito, F (1998). The role of chromosomal change in speciation. In Endless Forms: Species and Speciation, DJ Howard and SH Berlocher, eds. Oxford University Press.