Somatic mutation as a factor influencing the lengths of the haploid and diploid phases of sexual life cycles
The life cycle of a sexually-reproducing organism involves an alternation of generations between haploid and diploid stages (Mable & Otto 1998). The relative importance of these two life cycle stages varies between types of organism (Otto & Marks 1996), with some undergoing meiosis almost immediately after fertilisation and thus existing predominantly in the haploid form (haplonty), while others spend the greater part of their life cycles as diploids and produce only short-lived haploid gametes (diplonty). There are also organisms in which the haploid and diploid stages are of roughly equal significance (Mable & Otto 1998). The lengths of the haploid and diploid stages in an organism's life cycle are of profound evolutionary importance, since they determine whether it will undergo natural selection and mutation predominantly as a haploid or as a diploid. However, it has proved difficult to come up with broadly-applicable theories to explain the relative advantages of haplonty and diplonty, and to explain why some organisms adopt one strategy while other (sometimes quite closely related) species adopt another (Orr 1995, Gorshkov & Markar'eva 1999, Zeyl 2004).
One interesting explanation for the varying importance of diploidy in organisms' life cycles involves somatic mutations. These are the mutations that occur in the body cells of a multicellular organism, rather than in its germ line, and are therefore not passed on to its descendants (Orr 1995, Gorshkov & Makar'eva 1999). The effect of a single somatic mutation on an individual's fitness is likely to be small, because somatic mutations typically arise late in life and affect only a small part of the body. However, the sheer number of such mutations that occur during the development of a large organism could contribute to a significant overall loss of fitness (Orr 1995), and it is occasionally possible for somatic mutations to harm an organism drastically, notably by causing cancer (Otto & Orive 1995). It is hypothesised that diploids are less likely than haploids to suffer loss of fitness as a result of somatic mutations, since the effects of a mutation in one of their genes will be masked by the presence of a second, functioning copy of the gene (Mable & Otto 1998). This assumes that the majority of mutations are deleterious, and to some extent recessive - assumptions that are supported by theoretical and empirical evidence in many groups of organisms (Otto & Orive 1995, Zeyl 2004). If this is the case, we would expect the importance of diploidy to an organism to be related to its vulnerability to damage by somatic mutations.
The predominance of diploidy in large, complex organisms (such as animals and flowering plants) has been seen as evidence for the role of somatic mutation in the evolution of diploidy (Orr 1995, Otto & Orive 1995). It can be shown mathematically that number of somatic mutations suffered by an individual, and the potential loss of fitness that results from them, increase with an organism's size and lifespan, so large body size should favour diploidy (Orr 1995, Gorshkov & Markar'eva 1999). In simple multicellular organisms, 'intraorganismal selection' between competing cell lines has the potential to mitigate the harm caused by somatic mutations, since healthy cell lines can substitute for mutated ones during development. In species with more complicated, highly pre-determined patterns of development, however, the potential for intraorganismal selection of this kind is limited and somatic mutations will thus have a greater deleterious effect on overall fitness, increasing the selective advantage of being diploid (Otto & Orive 1995). This may explain the fact that animals, which are developmentally complex, are almost obligatory diploids (with the exception of certain invertebrates in which males are haploid), while large multicellular algae, which have far simpler development patterns, are often haploid (Gorshkov & Markar'eva 1999).
Somatic mutation provides an attractive explanation for the evolution of diplonty, because it is supported by mathematical models (Orr 1995, Otto & Orive 1995), and because it provides a mechanism by which natural selection acting on individuals could favour diploidy, whereas some alternative theories (such as the idea that diploidy speeds up evolution) posit group selection (Zeyl 2004). However, somatic mutation cannot be the whole story. Firstly, it fails to explain the occurrence of diplonty in some unicellular organisms, which are not subject to somatic mutations. Nor can it account for diplonty in polyploid organisms (such as certain red algae), in which the entire genome has been duplicated, so that deleterious mutations ought to be masked by healthy copies of a gene regardless of whether or not a cell is diploid (Mable & Otto 1998).
Gorshkov, V.G. and A.M. Makar'eva (1999). Haldane's Rule and Somatic Mutations. Russian Journal of Genetics 33: 611-617.
Mable, B.K. and S.P. Otto (1998). The evolution of life cycles with haploid and diploid phases. BioEssays 20: 453-462.
Orr, H.A. (1995). Somatic mutation favours the evolution of diploidy. Genetics 139: 1441-1447.
Otto, S.P. and J.C. Marks (1996). Mating systems and the evolutionary transition between haploidy and diploidy. Biological Journal of the Linnean Society 57: 197-218.
Otto, S.P. and M.E. Orive (1995). Evolutionary consequences of mutation and selection within an individual. Genetics 141: 1173-1187.
Zeyl, C. (2004). Experimental studies of ploidy evolution in yeast. FEMS Microbiology letters 233: 187-192.
This was originally written as a university biology essay