Chromosome Synapsis and Recombination in Male-Sterile and Female-Fertile Interspecies Hybrids of the Dwarf Hamsters (Phodopus, Cricetidae)

Abstract

Hybrid sterility is an important step in the speciation process. Hybrids between dwarf hamsters Phodopus sungorus and P.campbelli provide a good model for studies in cytological and genetic mechanisms of hybrid sterility. Previous studies in hybrids detected multiple abnormalities of spermatogenesis and a high frequency of dissociation between the X and Y chromosomes at the meiotic prophase. In this study, we found that the autosomes of the hybrid males and females underwent paring and recombination as normally as their parental forms did. The male hybrids showed a significantly higher frequency of asynapsis and recombination failure between the heterochromatic arms of the X and Y chromosomes than the males of the parental species. Female hybrids as well as the females of the parental species demonstrated a high incidence of centromere misalignment at the XX bivalent and partial asynapsis of the ends of its heterochromatic arms. In all three karyotypes, recombination was completely suppressed in the heterochromatic arm of the X chromosome, where the pseudoautosomal region is located. We propose that this recombination pattern speeds up divergence of the X- and Y-linked pseudoautosomal regions between the parental species and results in their incompatibility in the male hybrids.

Keywords: MLH1; SYCP3; hybrid sterility; pseudoautosomal region; sex chromosomes; synaptonemal complex; γH2A.X.

Figure 1

Histological sections of testes of Phodopus campbelli (a–c) and F1 type A (d,e) and type B (f–l) hybrids stained by hematoxylin-eosin. D, diplotene-diakinesis; DEBR, cellular debris; M, metaphase I and II; P, pachytene; SER, Sertoli cell; SPG, spermatogonium; SPTD, spermatid; SPZ, spermatozoid; E, early spermatocyte I at leptotene-zygotene. (a) tubules at different stages of the seminiferous epithelium cycle (V, VII and XII) showing an undisturbed progression of the spermatogenic wave; (b) stage VIII tubule containing Sertoli cells, spermatocytes I at pachytene and spermatids, its lumen containing mature spermatozoa; (c) stage XIII-XIV tubule containing spermatocytes at metaphase I and II and spermatids at the upper layer and Sertoli cells and spermatogonia at the lower layer; (d) aberrant seminiferous tubules without spermatogenic epithelium, (e) wall of an empty tubule with dividing spermatogonia and Sertoli cells only; (f) tubule with an excess of early spermatocytes I at leptotene and zygotene at the lower layer, cellular debris at the upper layer; (g) tubule with an excess of spermatocytes I at pachytene; (h) tubule wall with an excess of spermatocytes I at diplotene-diakinesis; (i) tubule containing degenerating pachytene spermatocytes and cellular debris; (j) tubule containing degenerating diplotene-diakinesis spermatocytes and cellular debris; (k) tubule with abnormal spermatids in the wall and abnormal spermatozoa in the lumen; (l) abnormal spermatozoa in the epididymal smear. Bar: 20 µm.

Figure 2

Pachytene spermatocytes (a–c) and oocytes (d–f) of P. sungorus (a,d), P. campbelli (b,e) and F1 hybrids (c,f) after immunolocalisation of SYCP3 (red), MLH1 (green) and centromeric proteins (blue). Arrowheads show misaligned centromeres at XX bivalents. Bar: 10 µm.

Figure 3

Distribution of MLH1 foci along identifiable bivalents from pachytene spermatocytes and oocytes of P. sungorus, P. campbelli and F1 hybrids. The bivalents are indicated on the left-hand side of the graphs. Centromere positions are shown by arrowheads. The X axis shows the position of MLH1 foci at the bivalent. The marks on the X axis are separated by approximately 1 µm of the SC length. Stacked columns show the frequency of the bivalents containing one (blue), two (yellow) and three (violet) MLH1 foci within each interval. The figures in the legends below each plot show the number of chromosomes/MLH1 foci plotted.

Figure 4

Electron microphotographs of pachytene spermatocytes of P. sungorus (a) and F1 hybrids (b–d) after AgNOR staining. Letters indicate the distal ends of the sex chromosome axes. (a) complete synapsis between Xp and Yq; (b) association between misaligned ends of Xp and Yq; (c) asynapsis of X and Y; (d) asynapsis of X and Y with the Y axis forming self-synapsis fold-back. Bar: 10 µm.

Figure 5

Pachytene spermatocytes (a,b) and oocyte (c) of P. sungorus (a) and F1 hybrids (b,c) after immunolocalisation of SYCP3 (green), and γH2A.X (red). Bottom panel (d–f) shows zoom of the upper white ellipses. (a,d) completely synapsed XY bivalent. Asynapsed parts of Xq and Yp are labelled with γH2A.X. Arrowhead indicates the unlabelled pairing region; (b,e) co-localised asynapsed X and Y axes in a shared cloud of γH2A.X-positive chromatin. Arrowhead indicates Y axis; (c,f) XX bivalent with asynapsed Xp. Arrowheads indicate the ends of asynapsed axes. Bar: 10 µm.