Genome elimination from the germline cells in diploid and triploid male water frogs Pelophylax esculentus

Abstract

Hybridogenesis is a hemiclonal reproductive strategy in diploid and triploid hybrids. Our study model is a frog P. esculentus (diploid RL and triploids RLL and RRL), a natural hybrid between P. lessonae (LL) and P. ridibundus (RR). Hybridogenesis relies on elimination of one genome (L or R) from gonocytes (G) in tadpole gonads during prespermatogenesis, but not from spermatogonial stem cells (SSCs) in adults. Here we provide the first comprehensive study of testis morphology combined with chromosome composition in the full spectrum of spermatogenic cells. Using genomic in situ hybridization (GISH) and FISH we determined genomes in metaphase plates and interphase nuclei in Gs and SSCs. We traced genomic composition of SSCs, spermatocytes and spermatozoa in individual adult males that were crossed with females of the parental species and gave progeny. Degenerating gonocytes (24%–39%) and SSCs (18%–20%) led to partial sterility of juvenile and adult gonads. We conclude that elimination and endoreplication not properly completed during prespermatogenesis may be halted when gonocytes become dormant in juveniles. After resumption of mitotic divisions by SSCs in adults, these 20% of cells with successful genome elimination and endoreplication continue spermatogenesis, while in about 80% spermatogenesis is deficient. Majority of abnormal cells are eliminated by cell death, however some of them give rise to aneuploid spermatocytes and spermatozoa which shows that hybridogenesis is a wasteful process.

Formation of hemiclonal reproduction and hybridogenesis inPelophylax waterfrogs studied with species-specific cytogenomic probes

“…We were, therefore, interested in whether the synthesized P. esculentus progeny eliminates preferentially parental genomes of P. ridibundus or P. lessonae . In all but one case P. esculentus tadpoles eliminated mostly lessonae genome and retained ridibundus genome in their germ cells, well in agreement with previous studies (Chmielewska et al, 2022; Dedukh et al, 2019; Dedukh & Krasikova, 2022). Neither the direction of the cross nor the origin of maternal mtDNA had any effect on an overrepresentation of ridibundus hemiclones.…”

“…Based on the analysis of micronuclei, it has been suggested that P. lessonae chromosomes were eliminated. Yet, it has never been confirmed (Chmielewska et al, 2022;Dedukh et al, 2019Dedukh et al, , 2020. Here, using a newly developed marker specific for P.…”

“…Based on the analysis of micronuclei, it has been suggested that P. lessonae chromosomes were eliminated. Yet, it has never been confirmed (Chmielewska et al, 2022; Dedukh et al, 2019, 2020). Here, using a newly developed marker specific for P. lessonae chromosomes, we identified P. lessonae chromosomes both in micronuclei and in standard cell nuclei during interphase for the first time.…”

“…Svinin et al, 2013), and not the R-E or pure P. esculentus populations. Since all P. esculentus newborns were diploid and originated from two haploid parental gametes, the L-E system with the occurrence of diploid taxa seems to be evolutionarily ancestral to the L-E system with the occurrence of polyploid P. esculentus , and to the pure P. esculentus populations that are likely derived from the L-E system with polyploids (Chmielewska et al, 2022; Christiansen, 2005, 2009; Dedukh et al, 2022).…”

“…Schmidt et al, 2011; Tunner & Heppich, 1981). In the so far investigated hybridogenetic species, genome elimination occurs gradually during the gonocyte proliferation in the early embryonic development and it does not occur in adult individuals (Chmielewska et al, 2022; Dedukh et al, 2019, 2020; Majtánová et al, 2021). Genome elimination in diploid and triploid hybrids from nature (Chmielewska et al, 2022; Dedukh et al, 2019, 2020) as well as in laboratory F1 hybrids (Dedukh et al, 2019), this study) occurs gradually and involves micronuclei formation.…”