The properties of meiotic gene conversion important in its effects on evolution

Abstract

SUMMARYIn order to determine how important gene conversion is as a force in populations, extensive surveys have been made to determine the limits, distributions and typical values of evolution-related conversion parameters from various fungi, Drosophila and maize. The conversion frequency, c, had an enormous range for different mutations within a species, with loci often having different means and limits for c. b, the frequency of a particular allelle in the products of meiotic tetrads or octads with aberrant segregation ratios, often showed values near the theoretical extremes; d, disparity in direction of conversion, was frequent and often extreme. b and d could only be studied in fungi, and their less extreme values in Saccharomyces than in Sordaria orAscobolus are clearly related to the lack of frame-shift mutations in the yeast data. For different mutations at a locus, neither c nor b showed normal distributions; both gave dispersed distributions, sometimes multi-modal for b; c and b were not usually correlated. y, the force of meiotic gene conversion on allele frequencies, had a large range of positive and negative values, with different loci often having quite different limits and mean absolute values. For different mutations at a locus, y showed dispersed distributions, with little tendency to cluster around zero. The high proportion of non-zero y values in all fungi where this could be studied, with large departures from zero being most extreme in Ascobolus and Sordaria brevicollis, shows that gene conversion could often be an important force in changing allele frequencies in favour of mutant or wild-type in these organisms. Even in organisms with much lower c or absolute d values, conversion could still be important, depending on dominance, selection coefficients and mutation rates. Presumed frame-shifts tended to have more extreme disparity and hence higher absolute y values than did presumed base-substitutions, but with no Consistent C differences. Induced mutations had similar or slightly higher absolute y values compared with spontaneous mutations.

GC Content Evolution of the Human and Mouse Genomes: Insights from the Study of Processed Pseudogenes in Regions of Different Recombination Rates

“…Two other models predict that variations in GC content are due to bias in fixation of AT toward GC mutations because of selection for GC (Bernardi and Bernardi 1986;Charlesworth 1994;Eyre-Walker 1999;Bernardi 2000) or a neutral process called GC-biased gene conversion (GC-BGC) (for review see Eyre-Walker 1993;Galtier et al 2001;Smith et al, 2002;Webster et al, 2003;Galtier 2003;Marais 2003). In meiotic recombination, parental chromosomes form heteroduplexes (DNA with one strand from the male and one strand from the female) in which there can be AT/GC heteromismatches (Lamb 1984). If subjected to the effect of GC-BGC, these mismatches are preferentially repaired in GC in mammalian genomes (Brown and Jiricny 1998;Bill et al 1998;Kudla et al 2004).…”

Tests of double-strand gap repair as a major source of meiotic gene conversion in fungi

“…Lamb (1984Lamb ( , 1986 having analysed several sets of data, concluded that significant disparity is not uncommon in yeast for base substitutions or for large deletions, and that the amount of disparity in yeast was very similar to that in Ascobolus and Sordaria when comparable types of mutations were studied. Some of the yeast data of Fogel et al (1979) underestimate the disparity which would be found in + x m crosses as the data for arg4 mutations came wholly or largely from heteroallelic crosses, sometimes from pooled data from heteroallelic crosses of reversed phase (e.g., arg4 16, +/ +, 17 and 16, 17/ + ,+) where co-correction could greatly reduce observed disparities.…”

“…Criteria 6 and 7, table 1, relate to the presence of parity or disparity in conversion direction, with very frequent and often extreme disparity in Ascobolus and Sordaria (details given by Lamb, 1984Lamb, , 1986. Szostak et a!.…”

Gene conversion disparity in yeast:its extent, multiple origins, and effects onallele frequencies

“…In a heteroallelic cross, conversions at any site are usually a mixture of conversions involving only that site, with its own conversion properties, and of co-conversions in which correction could be triggered by the other nearby heterozygous site, possibly reducing overall disparity for that site: in Table 2, arg4-16 in a monohybrid cross had ǹ40% 6:2/2:6 disparity, but this was reduced in heteroallelic crosses with either or both arg4-17 and arg4-19 to ǹ12, ǹ17 or ǹ25%. Data from filamentous fungi were nearly all from monohybrid crosses (Lamb, 1984).…”

“…The earlier yeast data were analysed for c, d and y by Lamb (1984Lamb ( , 1985Lamb ( , 1986, so only newer data are covered in Table 6, using only cases with 20 or more conversion asci. Conversion frequencies were high, with c of 0.156-0.548, for bik1 and the nearby his4, and for MAT, but lower for ARG4 RV with different homozygous deletions, 0.036-0.091.…”