NOTES ON THE DATA
Bacillus data (Zawadzki et al. 1995). Compatibility was estimated via interstrain transformation of Bacillus isolates, mostly obtained from the wild. Compatibility values were standardized by reference to within-strain transformation compatibility to give a relative measure with 100% compatibility at 0% DNA divergence. Transformation efficiency was measured by testing the ability of the donor strain to transmit antibiotic resistance to an antibiotic-sensitive strain. Dilution of donor DNA had little effect, but recipient strains from the wild were somewhat variable in transformation probability, and strains with restriction enzymes were particularly slow to transform, presumably because donor DNA was susceptible to restriction enzyme cutting. We excluded such strains from the analysis. The ability to transform in prokaryotes is somewhat similar to eukaryotic prezygotic compatibility, particularly in its dependence on mismatch repair (mismatch repair activity leads to lowered compatibility, as in yeast, below). Mismatch repair strongly enhances sexual isolation in E. coli, but has little effect in Bacillus; it is thought that sequence divergence in Bacillus lowers recombination directly because of a reduction in tendency to form heteroduplex DNA molecules during transformation (Majewski and Cohan 1998). However, as transformation efficiency is measured following survival of the transformed progeny, it may contain elements of “postzygotic” as well as “prezygotic” compatibility. DNA divergence was measured by the authors on a panel of genes (Zawadzki et al. 1995).
Saccharomyces (Liti et al. 2006). Crosses within and between a number of yeasts of the genus Saccharomyces were performed, and spore viability was assessed as a measure of compatibility. Recombination requires sequence similarity, and is necessary for successful chromosomal pairing in hybrid Saccharomyces, and so diploid hybrids between divergent populations or species tend to be sterile due to meiosis failure. When mismatch repair is inactivated, fertility improves, suggesting that a direct effect of sequence divergence via its interaction with mismatch repair is the cause of incompatibility (Hunter et al. 1996; Greig et al. 2003). Recombination in both meiotic and mitotic repair declines approximately exponentially with yeast sequence divergence (Chen and Jinks-Robertson 1999); these results suggest that a simple linear or first-order response of mismatch incompatibility to sequence divergence is likely.
Leptasterias starfish (Foltz 1997). Individuals of these starfish were sampled in areas in which a number of cryptic species co-occur. The numbers of F1 hybrids (identified by allozyme genotypes) sampled in nature divided by the number of individuals of the relevant pure species is the measure of compatibility used here. The hybrid frequencies measured may incorporate postzygotic as well as prezygotic effects on hybrid number.
Alpheus shrimps (Knowlton et al. 1993). Behavioral compatibility of shrimps was measured in a series of experiments in which aggressive and apparently sociable behaviors were scored, and a median compatibility score was constructed ranging from 1 (conspecific compatibility) to zero (all aggressive and no sociable behaviors). Because it refers only to presexual behaviors, and only 1% of heterospecific crosses actually produced fertile egg clutches, these values are likely to be overestimates of overall prezygotic compatibility. In these data, one of the crosses produced a behavioral compatibility value that was higher than the intraspecific values, giving a relative compatibility > 1. Genetic divergence was measured in two different ways: (1) via allozyme divergence (Nei's D), and (2) via % mtDNA (CoI) divergence.
Drosophila (Coyne and Orr 1989, 1997). Compatibility was estimated from Coyne and Orr's measures of reproductive isolation as 1− (reproductive isolation). Measures of reproductive isolation are of two types. (1) Prezygotic isolation, measured as 1−(frequency of heterospecific matings)/(frequency of within-species matings) in various types of choice or no-choice tests. When the frequency of heterospecific matings was greater than the frequency of homospecific matings (i.e., a negative index was obtained), the index was rounded to 0. (2) For postzygotic isolation a discrete measure was used. If any sex of F1 hybrid offspring of a single direction of cross between two species A and B was completely sterile or inviable, reproductive isolation was incremented by 0.25. Reciprocal crosses (i.e., A male × B female, versus B male ×A female) may yield different results. Thus, the value for postzygotic isolation varies from 0 (no sex in either reciprocal cross inviable or infertile) to 1 (both males and females sterile or inviable in both directions of cross), but can only take the values 0.00, 0.25, 0.50, 0.75, and 1.00.
Lepidoptera (Presgraves 2002). Genetic distance was based on Nei's D value obtained from studies of at least 13 allozyme loci. In the absence of allozyme studies, Nei's D was estimated via mtDNA divergence, and converted to Nei's D using a regression of Nei's D on mtDNA distance (Presgraves 2002). Postzygotic isolation was measured using a method similar to that of Coyne and Orr (1989), and we again estimated compatibility as 1− (reproductive isolation). Presgraves provides two overlapping datasets for postzygotic isolation: hybrid inviability and total postzygotic isolation, both of which we analyze. There were only 13–18 allopatric species pairs for each of these two postzygotic datasets, and as neither Presgraves nor we found any major differences when analyzing sympatric and allopatric species separately, we treated the sympatric and allopatric species together as a single dataset in our analyses. Total reproductive isolation was probably more reliable than the inviability measure, as only four crosses produced completely inviable progeny, and Presgrave's inviability measure averaged only 0.122, whereas average total reproductive isolation was 0.647.
Frogs (Sasa et al. 1998). Various measures of compatibility and isolation were presented by the authors. However, we use only two: (1) a combination of their egg hatch (EH) and metamorphosis rate (MET), in the form of EH×MET, which gives a compatibility index of survival from egg laying to metamorphosed adult; (2) a measure of compatibility equal to 1 −IPO2, where IPO2 is their discrete measure of postzygotic reproductive isolation. The EH and MET survival values, and resulting compatibility measure, are continuous measures, but IPO2 is an index of hybrid inviability and sterility in discrete units of 0.5 for a single direction of cross, a measure with discrete values 0, 0.5, and 1, somewhat like that used as an isolation index for Drosophila (Coyne and Orr 1997). In cases in which there was a measure for IPO1 (the average of IPO2 for both reciprocal cross directions, with discrete values 0, 0.25, 0.5, 0.75, and 1), we reconstructed the missing IPO2 measure so that we were able to use two values of IPO2 as suggested in Sasa et al. (1998). For genetic distance, we used the values of Nei's D from the same source.
Birds (Price and Bouvier 2002). For our genetic distance measures, we used data for HKY-corrected mtDNA divergence, and the Sibley DNA–DNA hybridization measure of ΔT50H. A single measure of compatibility was employed, based on the authors’ isolation index. Their “fertility” index, F, is like the index of Coyne and Orr, a discrete measure based on the sexes in reciprocal crosses that are inviable or infertile. F ranges from 1 to 5 in units of 0.5, with 1 indicating viability and fertility of all hybrids, 5 indicating complete inviability of all hybrids. For our purposes, we also interpret Price and Bouvier's values of F= 1* to be equivalent to 1.25, and their F= 5* to be equivalent to 4.5. The compatibility measure we use is then C= 1 − (F− 1)/4, which standardizes the measure on a scale of 0 to 1. Thus compatibility may occur in discrete units of 0, 0.125, 0.25, 0.375, 0.5, 0.625, 0.75, 0.875, 0.9375, and 1.000. An essentially complete absence of gene flow is expected from any Price and Bouvier index, F≥ 3, equivalent to compatibility C≤ 0.5 (all hybrids viable but infertile); thus our compatibility index overestimates overall hybrid fitness somewhat in the lower compatibility ranges, which may tend to make the fit more snowball-like.