Genomic compatibility between two phyllotine rodent species evaluated through their hybrids

In order to investigate the genomic compatibility between allopatric rodent species, Phyllotis darwini and Phyllotis magister, we have studied several cytogenetic and reproductive features of their laboratory hybrids. Of thir ty-one pairings between species, only five were successful, producing eleve n newborns. Like parents. hybrids had 38 metacentric chromosomes, except fo r the subtelocentric Y chromosome inherited from P. magister. There was alm ost total C and G band correspondence between homeologous autosomes. Howeve r, parental sex chromosomes had different morphology, C and G bands. Ag-NOR bands appeared as small telomeric Ag+ regions, distributed in four chromos omal pairs of darwini, three of magister and four homeologous chromosomes o f the hybrids. The three forms had similar indexes of NOR activity per cell , in spite of the variability in NOR expression which was always detected. Usually, only one member of parental homologous chromosomes showed AgNOR+; nevertheless, both homeologous chromosomes were active in many hybrid cells . The frequencies of cells that expressed their ribosomal genes in the two homologous or homeologous NOR chromosomes were similar in parental and hybr id cells. These results strongly suggest that ribosomal genes of both paren tal genomes would function codominantly in the hybrids. The gonad histologi cal and morphometric analyses showed that hybrids conformed to Haldane's ru le, since females were fertile and males were infertile. Our results indica te that P. darwini and P. magister genomes can Function in relative harmony and compatibility when they are placed together in their laboratory genera ted hybrids, suggesting that these species have few generic differences, pr obably because they have recently diverged.