Chromosome Asynapsis Is the Main Cause of Male Sterility in the Interspecies Hybrids of East Asian Voles (Alexandromys, Rodentia, Arvicolinae)

Abstract

Closely related mammalian species often have differences in chromosome number and morphology, but there is still a debate about how these differences relate to reproductive isolation. To study the role of chromosome rearrangements in speciation, we used the gray voles in the Alexandromys genus as a model. These voles have a high level of chromosome polymorphism and substantial karyotypic divergence. We investigated testis histology and meiotic chromosome behavior in the captive-bred colonies of Alexandromys maximowiczii, Alexandromys mujanensis, two chromosome races of Alexandromys evoronensis, and their interracial and interspecies hybrids, to explore the relationship between karyotypic differences and male hybrid sterility. We found that the seminiferous tubules of the males of the parental species and the interracial hybrids, which were simple heterozygotes for one or more chromosome rearrangements, contained germ cells at all stages of spermatogenesis, indicating their potential fertility. Their meiotic cells displayed orderly chromosome synapsis and recombination. In contrast, all interspecies male hybrids, which were complex heterozygotes for a series of chromosome rearrangements, showed signs of complete sterility. Their spermatogenesis was mainly arrested at the zygotene- or pachytene-like stages due to the formation of complex multivalent chains, which caused extended chromosome asynapsis. The asynapsis led to the silencing of unsynapsed chromatin. We suggest that chromosome asynapsis is the main cause of meiotic arrest and male sterility in the interspecies hybrids of East Asian voles.

Keywords: chromosomal polymorphism; crossing over; gray voles; hybrid sterility; synaptonemal complex.

Figure 1

Histological sections of testes of adult purebred A. evoronensis “Argi”, EVA (A,B), interracial hybrid A. evoronensis “Argi” × A. evoronensis “Evoron”, EVA × EVE (C,D), interspecies hybrids A. mujanensis × A. maximowiczii, MUJ × MAX (E,F), and A. maximowiczii × A. evoronensis “Evoron”, MAX × EVE (G,H) after staining by hematoxylin–eosin (A,C,E,G) and detection of apoptotic cells using TUNEL (green) and DAPI counterstaining (blue) (B,D,F,H). SPG, spermatogonium; SPC, spermatocyte; SPTD, spermatid; SPZ, spermatozoon. Bar: 20 μm.

Figure 2

Expected synaptic configurations in the parental species or races and their F1 hybrids: simple and complex heterozygotes.

Figure 3

Pachytene spermatocytes of A. maximowiczii, MAX (A–C), A. mujanensis, MUJ (D), A. evoronensis “Argi”, EVA (E,F), and A. evoronensis “Evoron”, EVE (G,H) homozygous (A,D,E,G) and heterozygous (B,C,F,H) for chromosome rearrangements after immunolocalization of SYCP3 (red), MLH1 (green), and centromeric proteins (blue). Arrowheads show the centromeres of heteromorphic bivalents. Bar: 10 µm.

Figure 4

Pachytene and pachytene-like spermatocytes of A. maximowiczii, MAX (A–C), A. evoronensis “Argi” × A. evoronensis “Evoron”, EVA × EVE (D–F), A. mujanensis × A. maximowiczii, MUJ × MAX (G–I), A. evoronensis “Argi” × A. mujanensis, EVA × MUJ (J–L), and A. evoronensis “Argi” × A. maximowiczii, EVA × MAX (M–O) after immunolocalization of SYCP3, SYCP1, MLH1, γH2A.X, and centromeric proteins (ACA). Arrowheads show asynapsed SC regions of the autosomes. Bar: 10 µm.

Figure 5

Electron microphotographs of pachytene spermatocytes of interspecies hybrids A. mujanensis × A. maximowiczii, MUJ × MAX (A) and A. maximowiczii × A. evoronensis “Argi”, MAX × EVA (B) stained with silver nitrate. Lowercase letters mark heteromorphic synaptic configurations: univalents: a, d; multivalents: b, e–h; inversion loop and its schematic representation: c, c′. Bar: 5 µm.