Genomic effects on advertisement call structure in diploid and triploid hybrid waterfrogs (Anura, Pelophylax esculentus)

Abstract

Background: In anurans, differences in male mating calls have intensively been studied with respect to taxonomic classification, phylogeographic comparisons among different populations and sexual selection. Although overall successful, there is often much unexplained variation in these studies. Potential causes for such variation include differences among genotypes and breeding systems, as well as differences between populations. We investigated how these three factors affect call properties in male water frogs of Pelophylax lessonae (genotype LL), P. ridibundus (RR) and their interspecific hybrid P. esculentus which comes in diploid (LR) and triploid types (LLR, LRR).

Results: We investigated five call parameters that all showed a genomic dosage effect, i.e. they either decreased or increased with the L/R ratio in the order LL-LLR-LR-LRR-RR. Not all parameters differentiated equally well between the five genotypes, but combined they provided a good separation. Two of the five call parameters were also affected by the breeding system. Calls of diploid LR males varied, depending on whether these males mated with one or both of the parental species (diploid systems) or triploid hybrids (mixed ploidy systems). With the exception of the northernmost mixed-ploidy population, call differences were not related to the geographic location of the population and they were not correlated with genetic distances in the R and L genomes.

Conclusions: We found an influence of all three tested factors on call parameters, with the effect size decreasing from genotype through breeding system to geographic location of the population. Overall, results were in line with predictions from a dosage effect in L/R ratios, but in three call parameters all three hybrid types were more similar to one or the other parental species. Also calls of diploid hybrids varied between breeding systems in agreement with the sexual host required for successful reproduction. The lack of hybrid call differences in a mixed-ploidy population at the northern edge of the water frog distribution is likely to be associated with genetic particularities, including a) low genetic variability and/or b) a local loss of genes coding for genotype-dependent call differentiation under conditions where female discrimination between diploid and triploid males is not beneficial.

Figure 1

Overview of adult genotypes, gametes types (in circles) and resulting offspring. LL = P. lessonae, LR, LLR and LRR = P. esculentus. a) the L-E system, where only diploid hybrids are produced, and non-hybrid genotypes from matings between hybrids typically die prematurely. b) the E-E system, where all three hybrid types can cross, but only hybrid genotypes survive to reproductive maturity. In the L-E system, the R genome is never recombined, and L genomes are provided by P. lessonae. In the E-E system, three types of gametes are produced by hybrids and both L and R genomes regularly undergo recombination when they are present in double copy in triploid hybrids (Christiansen 2009).

Figure 2

Map showing geographical locations of recorded populations. Dots = diploid populations with diploid hybrids and one or both parental species; triangles = mixed ploidy populations with mainly diploid and triploid hybrids, sometimes in sympatry with very few P. lessonae. For absolute numbers see Table 1.

Figure 3

Measurements taken for call variables. (a) measurements on P. esculentus call (LR type), (b) single pulse group and pulse measurements, and (c) list of derived variables used for subsequent analyses.

Figure 4

Means (± 1 S.E.) of the five call parameters (panels a-e) defined in Figure 3for P. lessonae (LL), P. ridibundus (RR) and their interspecific diploid (LR) and triploid (LLR, LRR) hybrids P. esculentus. Sample sizes are given in Table 2.

Figure 5

Discriminant functions plot of water frog calls. Canonical scores 1 and 2 were calculated from the combined parameters CALLDUR, PGR, PPPGR, IPGRIP and 75PERC.

Figure 6

Cluster dendrogram calculated from Euclidean call distances (based on discriminant analysis scores) between genotypes from different populatio ns. Given in italics are species names, P. esculentus ploidy types (LRR, LR, LLR), and population systems within the LR group (Pps = polyploid system, DpS = diploid system). Two groups (LLR and LRR from the most northern population of Genarp) are underlined to indicate their exceptional behavior within the overall pattern of genotype-specific clustering.

Figure 7

Comparative overview of advertisement calls from Germany (Döbern) and Sweden (Genarp). Shown are representative oscillograms of calls by each hybrid type recorded at approximately 20°C.