Sister chromosome pairing maintains heterozygosity in parthenogenetic lizards

Abstract

Although bisexual reproduction has proven to be highly successful, parthenogenetic all-female populations occur frequently in certain taxa, including the whiptail lizards of the genus Aspidoscelis. Allozyme analysis revealed a high degree of fixed heterozygosity in these parthenogenetic species, supporting the view that they originated from hybridization events between related sexual species. It has remained unclear how the meiotic program is altered to produce diploid eggs while maintaining heterozygosity. Here we show that meiosis commences with twice the number of chromosomes in parthenogenetic versus sexual species, a mechanism that provides the basis for generating gametes with unreduced chromosome content without fundamental deviation from the classic meiotic program. Our observation of synaptonemal complexes and chiasmata demonstrate that a typical meiotic program occurs and that heterozygosity is not maintained by bypassing recombination. Instead, fluorescent in situ hybridization probes that distinguish between homologues reveal that bivalents form between sister chromosomes, the genetically identical products of the first of two premeiotic replication cycles. Sister chromosome pairing provides a mechanism for the maintenance of heterozygosity, which is critical for offsetting the reduced fitness associated with the lack of genetic diversity in parthenogenetic species.

Figure 1

Oocytes from parthenogenetic A. tesselata contain twice the amount of chromosomal DNA compared to sexual A. gularis. (a) DAPI-stained chromosomes in germinal vesicles (GVs) from A. gularis. 3D projections of four GVs are shown, details on size and quantifications are available as Suppl. Table 1. Scale bars correspond to 10 μm. (b) GVs from A. tesselata. (c) Quantification of chromosome volumes. (d) Quantification of DNA content in somatic cells by flow cytometry. Fluorescence intensities from biological triplicates of ∼50,000 cells were averaged and normalized against samples from A. gularis which was set at 100 to facilitate comparison.

Figure 2

Visualization of chiasmata and synaptonemal complexes in parthenogenetic A. tesselata and sexual A. tigris. Projection of bivalents from A. tigris (a) and A. tesselata (b) GVs in diplotene. Scale bars correspond to 10 μm. (c) EM image of A. tesselata GV in pachytene. Several sections of synaptonemal complexes are visible. Scale bar corresponds to 2 μm. Close-ups for two areas are shown in (d) and (e). A close-up of a SC from A. tigris is shown in (f). Scale bars correspond to 200 nm.

Figure 3

Meiosis in sexual and parthenogenetic Aspidoscelis species. In normal meiosis a single round of DNA replication is followed by two consecutive divisions that result in a haploid gamete and three polar bodies. Homologs are shown in red and blue. Recombination generates chimaeric chromosomes. Premeiotic doubling of chromosomes allows for pairing of homologous or sister chromosomes. Homolog pairing and recombination result in some loss of heterozygosity in the mature oocyte. Recombination between pairs of sister chromosomes maintains heterozygosity at all loci.

Figure 4

Internal telomeric repeats distinguish homologs in A. neomexicana and demonstrate sister chromosome pairing. (a) A CCCTAA₍₃₎ peptide nucleic acid probe (red) identifies chromosome termini and large internal telomere repeat regions on metaphase spreads of A. tigris chromosomes prepared from fibroblast cultures. DAPI stained chromosomes are shown in blue. (b) Chromosome terminal signals, but no nternal telomeric repeats are detected in A. inornata. (c) Chromosomes inherited from A. tigris but not from A. inornata are identified by internal telomeric repeats in A. neomexicana. (d) Projection of a subset of images from an A. neomexicana GV visualized by confocal microscopy. DAPI-stained chromosomes in white and the telomeric probe in red. (e) Close-up of four representative areas showing paired fluorescence signals. The differences in resolution stem from differences in projection angles.