In most organisms, cells typically maintain genome integrity, as radical genome reorganization leads to dramatic consequences. However, certain organisms, ranging from unicellular ciliates to vertebrates, are able to selectively eliminate specific parts of their genome during certain stages of development. Moreover, partial or complete elimination of one of the parental genomes occurs in interspecies hybrids reproducing asexually. Although several examples of this phenomenon are known, the molecular and cellular processes involved in selective elimination of genetic material remain largely undescribed for the majority of such organisms. Here, we elucidate the process of selective genome elimination in water frog hybrids from the Pelophylax esculentus complex reproducing through hybridogenesis. Specifically, in the gonads of diploid and triploid hybrids, but not those of the parental species, we revealed micronuclei in the cytoplasm of germ cells. In each micronucleus, only one centromere was detected with antibodies against kinetochore proteins, suggesting that each micronucleus comprises a single chromosome. Using 3D-FISH with species-specific centromeric probe, we determined the role of micronuclei in selective genome elimination. We found that in triploid LLR hybrids, micronuclei preferentially contain P. ridibundus chromosomes, while in diploid hybrids, micronuclei preferentially contain P. lessonae chromosomes. The number of centromere signals in the nuclei suggested that germ cells were aneuploid until they eliminate the whole chromosomal set of one of the parental species. Furthermore, in diploid hybrids, misaligned P. lessonae chromosomes were observed during the metaphase stage of germ cells division, suggesting their possible elimination due to the inability to attach to the spindle and segregate properly. Additionally, we described gonocytes with an increased number of P. ridibundus centromeres, indicating duplication of the genetic material. We conclude that selective genome elimination from germ cells of diploid and triploid hybrids occurs via the gradual elimination of individual chromosomes of one of the parental genomes, which are enclosed within micronuclei.

在大多数生物中，细胞通常会维持基因组的完整性，因为彻底的基因组重组会导致巨大的后果。但是，从单细胞纤毛虫到脊椎动物的某些生物能够在某些发育阶段选择性地消除其基因组的特定部分。而且，在无性繁殖的种间杂种中发生部分或完全消除亲本基因组之一。尽管已知了这种现象的几个例子，但对于大多数这类生物来说，仍未广泛描述选择性消除遗传物质的分子和细胞过程。在这里，我们阐明了从Pelophylax esculentus水蛙杂交中选择性基因组消除的过程通过杂交发生复杂的繁殖。具体而言，在二倍体和三倍体杂种的性腺中，而不在亲本物种的性腺中，我们揭示了生殖细胞的细胞质中的微核。在每个微核中，只有一个针对着线粒体蛋白的抗体检测到了一个着丝粒，这表明每个微核都包含一条染色体。使用3D-FISH与物种特异性着丝粒探针，我们确定了微核在选择性基因组消除中的作用。我们发现，在三倍体LLR杂种中，微核优先包含P. ridibundus染色体，而在二倍体杂种中，微核优先包含L. lessonae染色体。核中着丝粒信号的数量表明生殖细胞是非整倍体，直到它们消除了亲本物种之一的整个染色体组。此外，在二倍体杂种中，在生殖细胞分裂的中期观察到错位的假单胞菌染色体，这表明它们可能由于无法附着到纺锤体和正确分离而被消除。此外，我们描述了P. ridibundus着丝粒数量增加的生殖细胞，表明遗传物质重复。我们得出结论，从二倍体和三倍体杂种的生殖细胞中选择性清除基因组是通过逐渐消除封闭在微核内的亲本基因组之一的单个染色体而发生的。