Testing the translocation hypothesis and Haldane’s rule in Rumex hastatulus

Abstract

The translocation hypothesis regarding the origin of the XX/XY1Y2 sex chromosome system was tested with reference to the F1 hybrids between two chromosomal races of Rumex hastatulus. The hybrids derived from reciprocal crossing between the Texas (T) race and the North Carolina (NC) race were investigated for the first time with respect to the meiotic chromosome configuration in the pollen mother cells, pollen viability, and sex ratio. A sex chromosome trivalent in the NC × T males and two sex chromosome bivalents in the T × NC males were detected. The observed conjugation patterns confirmed the autosomal origin of the extra chromosome segments occurring in the North Carolina neo-sex chromosomes. Decreased pollen viability was found in the T × NC hybrid in contrast to the NC × T hybrid and the parental forms. Moreover, only in the T × NC hybrid sex ratio was significantly female-biased (1:1.72). Thus, Haldane's rule for both male fertility and male rarity was shown in this hybrid. According to the authors' knowledge, R. hastatulus is just the second plant with sex chromosomes in which Haldane's rule was evidenced.

Ancestral and neo‐sex chromosomes contribute to population divergence in a dioecious plant

“…Our findings, along with those of Kasjaniuk et al. (), raise the possibility that sex chromosome turnover may have contributed to the complete cessation of gene flow in the second stage of differentiation between R. hastatulus cytotypes. The most striking evidence in favor of this hypothesis is that our estimate for the time since the loss of recombination between the neo‐X and neo‐Y chromosomes overlaps with that for the loss of gene flow between cytotypes.…”

“…Nonetheless, the similar timescales we estimated for these events, combined with evidence from crossing studies that demonstrate the important effect of the rearrangement on cytotype hybrid fitness (Kasjaniuk et al. ), suggest a highly plausible relation between sex chromosome turnover, genetic divergence, and reproductive isolation between the cytotypes of R. hastatulus . Examination of genotypes in the contact zone could provide further insights into the relative role of geography and chromosomal rearrangement on allele frequency differentiation between R. hastatulus cytotypes.…”

“…The results of intercytotype crosses by Kasjaniuk et al. () suggest strong F 1 pollen sterility in crosses between XYY males and XY females, but not in the opposite direction. This asymmetrical crossing result is consistent with our evidence for greater historical gene flow from XY into the XYY clade and with historical gene flow of a Y haplotype into the southern XYY clade.…”

“…1B). Recent cytological studies using florescent probes to illuminate sex chromosome pairing during meiosis in hybrid cytotypes provide evidence confirming the X-autosome fusion model (Grabowska-Joachimiak et al 2015;Kasjaniuk et al 2018).…”

“…Recent work has demonstrated that male hybrids between the two cytotypes exhibit reduced fertility and viability (Kasjaniuk et al. ). Significantly, reduced pollen viability of F 1 male hybrids is asymmetric, occurring only when the XYY cytotype is the male parent for the hybrid.…”