Hybridogenesis in the Water Frogs from Western Russian Territory: Intrapopulation Variation in Genome Elimination

Abstract

Hybridogenesis in an interspecific hybrid frog is a coupling mechanism in the gametogenic cell line that eliminates the genome of one parental species with endoduplication of the remaining genome of the other parental species. It has been intensively investigated in the edible frog Pelophylax kl. esculentus (RL), a natural hybrid between the marsh frog P. ridibundus (RR) and the pool frog P. lessonae (LL). However, the genetic mechanisms involved remain unclear. Here, we investigated the water frogs in the western Russian territory. In three of the four populations, we genetically identified 16 RL frogs living sympatrically with the parental LL species, or with both parental species. In addition, two populations contained genome introgression with another species, P. bedriagae (BB) (a close relative of RR). In the gonads of 13 RL frogs, the L genome was eliminated, producing gametes of R (or R combined with the B genome). In sharp contrast, one RL male eliminated the L or R genome, producing both R and L sperm. We detected a variation in genome elimination within a population. Based on the genetic backgrounds of RL frogs, we hypothesize that the introgression of the B genome resulted in the change in choosing a genome to be eliminated.

Keywords: Pelopylax kl. esculentus; Serum albumin; cytochrome b; genome introgression.

Figure 1

Maps showing the collecting locations of frogs and the taxonomic identification tool. The bottom left map indicates the Russian Federation on Earth, and the upper map is a magnification showing the five collection locations included in four populations of the frogs used in this study (the two locations a and b are combined to one population P3 because they are located within a city and either do not include any P. kl. esculentus). The total number of collected frogs in each population is described after the name of location in parenthesis. The right bottom picture is a taxonomic identification tool, showing an electrophoretic pattern of amplified fragments of Serum albumin intron 1 (SAI-1) in the three different frog species. RR, marsh frog Pelophylax ridibundus; LL, pool frog P. lessonae; RL, edible frog P. kl. esculentus. R, SAI-1 fragment of P. ridibundus and L, SAI-1 fragment of P. lessonae. The maps were created using the SAS-Planet package.

Figure 2

Taxonomic composition of P. esculentus complex in the sampled populations. The composition ratios of P. lessonae, P. ridibundus and P. kl. esculentus are demonstrated in green, yellow and purple, respectively. P. kl. esculentus was identified in P1, P2 and P4 but not in P3.

Figure 3

A mitochondrial cytochrome b tree showing the mitochondrial origins of P. kl. esculentus, constructed using the Maximum Likelihood method (ML). Three large clades of haplogroups of P. lessonae, P. ridibundus and P. bedriagae are highlighted by boxes in green, yellow and red, respectively. The cytochrome b haplotypes of P. kl. esculentus from P4 were all included in the lessonae clade (green box), while those of P. kl. esculentus from P1 and P2 belonged to the lessonae, ridibundus or bedriagae clades. One ridibundus (P3–3) possessed the haplotype of P. bedriagae. The numbers at the nodes of the tree are the percentages of 500 bootstrap replicates.

Figure 4

Nuclear Serum albumin intron 1 tree in P. kl. esculentus and P. ridibundus, constructed using the Maximum Likelihood method. The two large clades of P. ridibundus and P. bedriagae are highlighted using a box of yellow and red, respectively. The SAI-1 haplotypes of P. kl. esculentus from P4 all belonged to the ridibundus clade, while those of P. kl. esculentus from P1 and P2 belonged to either clade. [CA]_n indicates the CA repeat (n, number of repeats) located at the 3′ flanking region of the transposon included in the ridibundus and bedriagae intron 1. The numbers at the nodes of the tree are the percentages of 500 bootstrap replicates.

Figure 5

Genome elimination in P. kl. esculentus. SAI-1 fragments amplified from finger (F) and gonad DNA (T, testis; O, ovary) in eight P. kl. esculentus from P4, three from P1 and four from P3 are shown. The L bands (lessonae) of these samples from gonads are all much weaker in depth while R bands (ridibundus) are slightly increased compared to those from the finger DNA, indicating L genome elimination in the gonads (a,b). On the other hand, one female and one male P. kl. esculentus from P1 and P2, respectively, did not show any difference in the depth of the SAI-1 bands between the finger and gonad DNA (b, boxed in yellow).

Figure 6

Diagram showing presumed hybridogenetic gametogenesis in P. kl. esculentus from the western Russian territory. P4 possesses an L-E system, where the L genome is eliminated in the testes of P. kl. esculentus and R sperm are produced under L mitochondria. Likewise, P1 and P2 had an L-E system, where the L genome is eliminated in the gonads and R/B gametes (combined genome of ridibundus and bedriagae) are produced only under the ridibundus mitochondria. In contrast, neither genome might be eliminated in one kl. esculents triploid female (P1–3) under bedriagae mitochondria. Furthermore, in the one kl. esculentus (P2–3) individual, the L or R genome is eliminated in the testis and the R and L sperm are produced under lessonae mitochondria. This represents genome elimination in the R-E system. Large circles indicate gonads of P. kl. esculentus, in which small circles are germ cells: mitochondria are shown by color of outline and nuclear genomes are shown by inner color or letter. Abbreviations are shown in the figure.