Polyploidization is a rare yet sometimes successful way for animals to rapidly create geno- and phenotypes that may colonize new habitats and quickly adapt to environmental changes. In this study, we use water frogs of the Pelophylax esculentus complex, comprising two species (Pelophylax lessonae, genotype LL; Pelophylax ridibundus, RR) and various diploid (LR) and triploid (LLR, LRR) hybrid forms, summarized as P. esculentus, as a model for studying recent hybridization and polyploidization in the context of speciation. Specifically, we compared the geographic distribution and genetic diversity of diploid and triploid hybrids across Europe to understand their origin, maintenance and potential role in hybrid speciation. We found that different hybrid and parental genotypes are not evenly distributed across Europe. Rather, their genetic diversity is structured by latitude and longitude and the presence/absence of parental species but not of triploids. Highest genetic diversity was observed in central and eastern Europe, the lowest in the northwestern parts of Europe. This gradient can be explained by the decrease in genetic diversity during postglacial expansion from southeastern glacial refuge areas. Genealogical relationships calculated on the basis of microsatellite data clearly indicate that hybrids are of multiple origin and include a huge variety of parental genomes. Water frogs in mixed-ploidy populations without any parental species (i.e. all-hybrid populations) can be viewed as evolutionary units that may be on their way towards hybrid speciation. Maintenance of such all-hybrid populations requires a continuous exchange of genomes between diploids and triploids, but scenarios for alternative evolutionary trajectories are discussed.
Study sites, sample sizes and population composition
Geographic coordinates, sample sizes and genotypic composition of water frog populations sampled for microsatellite analysis. Classification was based on microsatellite results and was complemented by additional information on the population through personal observation (e.g. on presence of parental genotypes even though none were captured for sampling). nL represents the number of individuals used for the L genome data set, nR stands for the number of individuals used for the R genome data set. N represents the number of individuals used in total from the respective population. Percentages account for the proportion of genotypes within N. Please note that the population marked with * is a subsample from a larger sample. Individuals from this population used in this study were selected to represent the proportion of genotypes in the larger original sample, which partly resulted in a deviation in sample size between nL and nR.
Hoffmann et al S1 (new).docx
Origin and number of frogs used for mtDNA analysis
Populations, geographic coordinates and sample sizes of samples used for mtDNA analysis. The column “Microsat No.*” refers to the numbering of populations in Table S1 (new), where only populations used in microsatellite analyses are listed. Types of mtDNA are abbreviated as follows: les = lessonae-type, rid = ridibundus-type, ber = bergeri-type, cf. bed = cf. bedriagae-type.
Hoffmann et al S2 (new).xlsx
Samples for microsatellite and mt DNA analysis
The Table summarizes which samples have been analyzed with microsatellites and/or mtDNA sequencing. Given are ID numbers, names and coordinates of samples sites an genotypes of frogs
Hoffmann et al S3 (new).xlsx
Microsatellite markers
This Table lists the alleles found in each sampled individual (specified by ID numbers) on the Basis of six microsatellite markers
Hoffmann et al S4 (new).xlsx
Genetic structure within the R and L genome
Genetic differentiation within the R and L genome of water frogs of the Pelophylax esculentus complex, according to Bayesian analyses implemented in the program STRUCTURE. The proportion of membership (parameter q according to Pritchard et al. 2000) of each individual in each of the two (R genome) and four (L genome) inferred clusters is shown.
Hoffmann et al S5 (new).xlsx
R-genome phylogeny (input)
Data input for R-Genome phylogeny in Fig. 6
Hoffmann et al S6a (new).txt
R-phylogeny (output)
Data Output for RgGenome phylogeny in Fig. 6
Hoffman et al S6b (new).nwk
L-genome phylogeny (input)
Data input for L-genome phylogeny in Fig. 7
Hoffmann et al S7a (new).txt
L-phylogeny (output)
Data output for L-Genome phylogeny in Fig. 7
Hoffman et al S7b (new).nwk
mtDNA sequences
Shown are mtDNAsequences for all individuals (characterized by ID numbers) that were anaylzed according to Table S3 (new).
Hoffmann et al S8 (new).nexus
mt DNA haplotypes across genotypes and populations
Supplement to Figure 8, showing the distribution of mtDNA haplotypes across genotypes and population types. Asterisks (*, **) indicate novel findings of the respective type of mtDNA in mixed-ploidy populations [24]* (Untermassfeld, Germany) and [52]** (Gaidary Iskov, Ukraine).
Hoffmann et al S9 (new).docx