Evolution of genomes in the hybridogenetic populations modelled by the Penna model

Background: Hybridogenesis is a very interesting example of reproduction which seems to integrate the sexual and clonal processes in one system. In a case of frogs, described in the paper, two parental species - Rana lessonae and Rana ridibunda can form fertile hybrid individuals - Rana esculenta. Hybrid individuals eliminate one parental haplotype from their germ line cells before meiosis (end before recombination) which implicates clonal reproduction of the haplotype transferred to the gametes. All three “species” are called “complex species”. To study the evolution of genomes in the hybridogenetic fraction of this complex species we have used the Monte Carlo based model rendering the age structured populations. The model enables the analysis of distribution of defective alleles in the individual genomes as well as in the genetic pool of the whole populations. Results: We have shown that longer isolation of hybrids’ populations leads to the speciation through emerging the specific sets of complementing haplotypes in their genetic pool. The fraction of defective alleles increases but the defects are complemented in the heterozygous loci. Nevertheless, even small supply of new hybrids generated by the two parental species or crossbreeding between hybrids and one of the parental species prevents the speciation and changes the strategy of the genome evolution from the complementing to the purifying Darwinian selection.

💡 Research Summary

The paper investigates the evolutionary dynamics of hybridogenetic populations using an age‑structured Monte Carlo simulation based on the Penna model. Hybridogenesis, exemplified by the European water frogs, involves two parental species—Rana lessonae and R. ridibunda—producing fertile hybrids (R. esculenta) that eliminate one parental haplotype from their germ line before meiosis. Consequently, the retained haplotype is transmitted clonally to gametes, while the somatic genome remains a heterozygous combination of both parental sets. This dual sexual‑asexual reproductive strategy creates a “complex species” in which gene flow and clonal inheritance coexist.

In the computational framework each individual’s genome is represented by a binary string (0 = functional allele, 1 = deleterious allele). Bits are expressed sequentially with age; when a predefined number of expressed deleterious bits (three in the simulations) is reached, the individual dies. Two parental species are initialized with distinct haplotypes of length 32 (or 64) bits. Hybrid individuals are generated by copying the entire haplotype of one parent (the “donor”) into the germ line and discarding the other parent’s haplotype before gametogenesis. Thus, hybrids reproduce sexually in terms of mating, but the transmitted genome is effectively a clone of the donor haplotype.

The authors explore two scenarios. In the first, a hybrid population is completely isolated after its formation. Under these conditions, the clonal haplotype accumulates deleterious mutations over generations because there is no recombination to purge them. However, because each hybrid’s somatic genome still contains the opposite parental haplotype, most deleterious alleles remain heterozygous and are phenotypically masked. The overall frequency of defective alleles in the population rises, yet mortality does not increase proportionally. Over many generations a set of complementary haplotypes becomes fixed—a “complementing haplotype pool” that allows the population to persist despite a high mutational load. This emergent structure is interpreted as a form of speciation driven by the hybridogenetic mode of inheritance.

In the second scenario, a small proportion (≈1 % per generation) of new hybrids is continuously introduced from the parental species, mimicking occasional back‑crosses or immigration. These newcomers bring different donor haplotypes, creating opportunities for recombination when hybrids mate. Recombination breaks up the previously complementary haplotype sets, exposing deleterious alleles in homozygous form. Consequently, natural selection acts as a purifying force: individuals carrying unmasked defects die, and the population’s mutational load is reduced. The system shifts from a “complementation‑driven” evolutionary regime to a classic Darwinian purifying selection regime. Even a modest influx of novel haplotypes is sufficient to prevent the fixation of the complementary haplotype pool and thus to halt the speciation process observed in the isolated case.

Key insights derived from the simulations are: (1) hybridogenesis can generate a stable genetic architecture that tolerates a high burden of recessive deleterious mutations through heterozygous complementation; (2) prolonged isolation of such a hybrid population can lead to the emergence of a distinct, self‑sustaining gene pool, effectively a new species; (3) gene flow, even at low levels, destabilizes this architecture, restoring the efficacy of purifying selection and maintaining genetic similarity with the parental species; (4) the Penna model’s age‑dependent expression of deleterious alleles captures realistic mortality patterns observed in amphibian populations, linking demographic ageing with genetic load.

The authors conclude that hybridogenetic systems occupy a unique niche between sexual and clonal reproduction, capable of both fostering rapid genomic divergence and being highly sensitive to introgression. Their work provides a quantitative framework for testing hypotheses about complex species formation, the balance between mutation accumulation and selection, and the role of occasional gene flow in shaping evolutionary trajectories. Future empirical work suggested includes whole‑genome sequencing of hybrid frog populations to detect complementary haplotype blocks and experimental manipulation of gene flow to validate the model’s predictions.