Differential Performance Among LDH-B Genotypes in Rana Lessonae Tadpoles

Abstract

Abstract. The European pool frog, Rana lessonae, is widely polymorphic for two common alleles (b, e) at the lactate dehydrogenase-B (LDH-B) locus. We compared fitness-related larval life-history traits among LDH-B genotypes, which originated from segregation in heterozygous parents, in an artificial pond experiment where tadpoles of R. lessonae from a Swiss population were raised together with tadpoles of the hemiclonal hybrid R. esculenta at two densities. In R. lessonae, LDH-B e/e homozygotes at each density had a higher proportion of metamorphs among survivors, reached metamorphosis earlier, and were heavier at metamorphosis than b/b homozygotes; b/e heterozygotes had intermediate values. That e/e individuals were superior to b/b in both time to and mass at metamorphosis is surprising because these two life-history traits are thought to reflect a performance trade-off; e/e genotypes apparently compensated for shorter time to metamorphosis by a higher growth rate. The two alleles showed the same performance ranking when combined in hybrids with a R. ridibunda allele: When R. esculenta from Swiss populations reared in the same ponds had received the e allele rather than the b allele from their R. lessonae parent, they reached metamorphosis earlier, but did not differ in mass at metamorphosis. The degree of linkage disequilibrium in the source population of the eight R. lessonae used as parents of the R. lessonae tadpoles is unknown, so we cannot exclude the possibility that the performance differences are caused by some anonymous tightly linked gene, rather than the LDH-B locus, that constitutes the genomically localized target of natural selection. A causal involvement of LDH-B is plausible, nevertheless, because this enzyme takes part in the central energy-metabolizing processes and has been reported to underlie fitness differences in other animals; also, differential performance of LDH-B genotypes has been observed in R. lessonae larvae from another population. The present results suggest strong directional selection for allele e; the sum of available data, including an independent laboratory experiment, suggests that partial environmentdependent overdominance combined with balancing selection favoring e/e homozygotes under some and b/b homozygotes under other conditions may be partially responsible for the broad maintenance of the LDH-B polymorphism in R. lessonae.

Genetic differentiation between alpine and lowland populations of a butterfly is related to PGI enzyme genotype

“…Interestingly, the expected and observed heterozygosity in L. tityrus is lower in alpine than in lowland populations. Low heterozygosity is often associated with fitness costs (Ochando and Ayala 1999, Hotz and Semlitsch 2000, Watt et al 2003. The results in L. tityrus are in contrast with altitudinal patterns in the dung fly Scathophaga cynipsea showing a higher genetic diversity at higher altitudes (Kraushaar et al 2002), and also with the intuitive assumption of a positive rather than a negative correlation with altitude, based on the harsher environmental conditions warranting high individual fitness.…”

“…Low heterozygosity is often associated with fitness costs (Ochando and Ayala 1999, Hotz and Semlitsch 2000, Watt et al 2003. Low heterozygosity is often associated with fitness costs (Ochando and Ayala 1999, Hotz and Semlitsch 2000, Watt et al 2003.…”

Experimental Hybridization as a Tool for Studying Selection in the Wild

“…1B), but the differences in phenotypic values were generally not interpreted in terms of genotypic differences. Clearly, the notion that hybrid fitness may best be evaluated at the level of individual hybrid genotypes has largely been overlooked, the only available examples being the studies by Hotz and Semlitsch (2000) in animals, and Lexer et al (2003) in plants. The latter study clearly demonstrates that evolutionary novelties such as rapid adaptation to a saline habitat can be readily achieved by the large genotypic variation underlying the relevant traits in segregating hybrid populations.…”

Microsatellites: A tool for evolutionary genetic studies of western Palearctic water frogs

“…Such genomically mapped loci will be used in analyzing hybridogenetic genome exclusion, and will permit consideration of physical linkage in analyses of microsatellite data from natural po-pulations. In addition, we will use microsatellites for cohort and individual identification in largescale experiments designed to complement existing data on differential performance of water frog genotypes (Semlitsch et al 1996(Semlitsch et al , 1997Schmidt et al 1998;Hotz et al 1999;Hotz & Semlitsch 2000). Finally, analyses of ridibunda haplotypes in F1-derived hemiclonal lineages that we are currently generating may provide general insights in tempo and mode of evolution of microsatellite markers.…”

“…we have primarily used electrophoretically analyzed protein loci (e.g. Uzzell & Hotz 1979;Hotz & Uzzell 1982: Hotz & Semlitsch 2000 and mitochondria1 DNA (Spolsky & Uzzell 1984, 1986Hotz et al 1992) as genetic markers. We presently are studying patterns of clonal diversity among R. escrilerttn lineages to test ideas about the evolutionary significance o f maintaining sexual recombination (Hotz et al 1994: Semlitsch et al 1996.…”