|MadSci Network: Genetics|
You've posed a couple of really big questions, so I will try to answer them in a logical order.
First, what is so special about diploidy? All mammals are diploid, therefore this is normal, right? Well, as you are aware many plants are polyploid: although rice is 2n, Wheat is 6n, and corn and most potatos are 4n. However, there are many examples of polyploids in the animal kingdom, too. Many amphibians are tetraploid, which is why frogs are a lousy genetic model organism despite their utility in studying development. Some of the racier salamanders are triploid, and female lines survive by parthenogenesis, or produce 3n eggs which can be fertilized by 1n sperm from diploid males of closely related species to yield 4n hybrids (1). Interestingly, closely related 2n and 4n species of salamanders have been described, where the polyploid salamanders are dwarves relative to their diploid cousins (2). In amphibians, polyploidy appears to be a frequent event, but does not account for the major divergence events within the Class.
Beyond the amphibians, polyploid survival has been demonstrated experimentally in clams, if not described in the wild (3). Some fish are polyploid: the Salmonidae family appears to have arisen by a 2n to 4n genome duplication event (4). Lungfish are also polyploid, with some of the largest genomes in the animal kingdom (up to 40X the size of the human genome)(6). Polyploidy has also been reported in several reptile species. Insects are even more fun, as some species have different ploidy by sex. For example, female bees are 2n, while male bees are 1n. Meanwhile almost all bacteria are effectively 1n.
So what is the big deal about diploidy? Well, the basic fact is that diploidy is not special, it just happens to be the system used in mammals. Because it is the genetic system in humans, it is generally taught as the "normal" case. The logical follow up question is: why are mammals so intolerant of polyploidy? Well, one hypothesis is that the establishment of sex chromosomes is the event which fixed the ploidy of birds and animals (7). Duplication of autosomes is not such a big deal because there is no mechanism for dosage compensation on such chromosomes. However, for sex chromosomes polyploidy has major consequences on sex determination (if all the tetraploids are male, propagation of tetraploid lines becomes a bit of a problem). Therefore once the sex chromosome system was established, genome duplication was no longer an option. Because the sex chromosomes were established very early in the evolution of these classes, the ploidy of the class was fixed at the ancestral level.
On to the second topic: Is 2n actually better than 1n because of recessive disease alleles? In organisms with long life spans and relatively few progeny, there is a major advantage in reducing the mutation rate to as low a level as possible, so that few progeny receive novel seriously deleterious mutations. One way to reduce the apparent mutation rate is to carry multiple copies of the genome. However, using the same logic 4n should be even better than 2n. I think that once the chromosome number was fixed by the evolution of sex chromosomes, if homologous sets of 4n chromosomes existed, they have subsequently diverged into separate pairs of 2n chromosomes.
Historically it has also been argued that 2n is better than 1n because recombination allows accelerated evolution to optimal allelic combinations, but the flip side of this argument is that recombination also breaks up optimal allelic combinations. Theoretical population geneticists have actually shown that if the recombination rate is allowed to vary in a 2n species, the fitness of a species optimizes at zero recombination once the optimal allelic combination exists(5). So why does recombination persist? The current hot theory is that recombination is simply a process evolved to help hold the chromosome pairs together at the metaphase plate so that the chromosomes will segregate properly into the gametes during meiosis, and that the genetic effects of this process are merely byproducts.
So how do polyploid organisms cope with extra copies of the genome? Although I can't tell you for sure, I can tell you that plants and other polyploids do not inactivate the extra chromosomes in the same fashion that female mammals inactivate extra copies of the X chromosome. The genetic regulatory mechanisms in polyploids simply appear to have adapted to the presence of extra chromosomal copies.
(1) Bogart et al, Temperature and sperm incorporation in polyploid salamanders, Science 1989 Nov 24;246(4933):1032-1034 (2) I wish the reference for this was better: I saw a lecture on the topic given by Scott F Gilbert in 1995 at Stanford, but cannot find any literature refs for you. (3) Guo et al, Sex determination and polyploid gigantism in the dwarf surfclam, Genetics 1994 Dec;138(4):1199-1206 (4) Griffiths, Miller, Suzuki, Lewontin and Gelbart: Genetic Analysis 5th edition p. 244-250 (5) Feldman et al, Population genetic perspectives on the evolution of recombination. Annu Rev Genet 1996;30:261-95 (6) Hinegardner R, Evolution of genome size, Molecular Evolution, FJ Ayala ed. p 179-199 (7) Ohno S, Sex Chromosomes and Sex Linked Genes, 1967 Springer-Verlag
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