Abstract
BackgroundInterspecies animal hybrids can employ clonal or hemiclonal reproduction modes where one or all parental genomes are transmitted to the progeny without recombination. Nevertheless, some interspecies hybrids retain strong connection with the parental species needed for successful reproduction. Appearance of polyploid hybrid animals may play an important role in the substitution of parental species and in the speciation process.ResultsTo establish the mechanisms that enable parental species, diploid and polyploid hybrids coexist we have performed artificial crossing experiments of water frogs of Pelophylax esculentus complex. We identified tadpole karyotypes and oocyte genome composition in all females involved in the crossings. The majority of diploid and triploid hybrid frogs produced oocytes with 13 bivalents leading to haploid gametes with the same genome as parental species hybrids usually coexist with. After fertilization of such gametes only diploid animals appeared. Oocytes with 26 bivalents produced by some diploid hybrid frogs lead to diploid gametes, which give rise to triploid hybrids after fertilization. In gonads of all diploid and triploid hybrid tadpoles we found DAPI-positive micronuclei (nucleus-like bodies) involved in selective genome elimination. Hybrid male and female individuals produced tadpoles with variable karyotype and ploidy even in one crossing owing to gametes with various genome composition.ConclusionsWe propose a model of diploid and triploid hybrid frog reproduction in R-E population systems. Triploid Pelophylax esculentus hybrids can transmit genome of parental species they coexist with by producing haploid gametes with the same genome composition. Triploid hybrids cannot produce triploid individuals after crossings with each other and depend on diploid hybrid females producing diploid eggs. In contrast to other population systems, the majority of diploid and triploid hybrid females unexpectedly produced gametes with the same genome as parental species hybrids coexist with.
Highlights
Interspecies animal hybrids can employ clonal or hemiclonal reproduction modes where one or all parental genomes are transmitted to the progeny without recombination
We have identified the genome composition of the oocytes in 3 diploid hybrid females used in crossing experiments and 3 triploid hybrid females that were not used in the crossings (Additional file 3: S1)
After analysis of 7 lampbrush chromosomal sets of diploid hybrid females we found oocytes with 13 bivalents corresponding to P. ridibundus chromosomes, oocytes with 26 bivalents and oocytes with 26 univalents, where 13 bi- or univalents corresponded to the chromosomes of P. ridibundus and the other 13 bi- or univalents corresponded to the chromosomes of P. lessonae (Additional file 3: Figure 1)
Summary
Interspecies animal hybrids can employ clonal or hemiclonal reproduction modes where one or all parental genomes are transmitted to the progeny without recombination. Appearance of polyploid hybrid animals may play an important role in the substitution of parental species and in the speciation process. Animal interspecies hybrids can develop clonal or hemiclonal reproduction modes where one or all parental genomes are transmitted to progeny without recombination [3, 4]. The role of the polyploid hybrids as intermediate stage towards independent species remains poorly understood. It is not known how polyploid and diploid interspecies animal hybrids appear and are maintained in the majority of population systems of clonally and hemiclonally reproducing species
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