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.
Highlights
Sex chromosomes in flowering plants are relatively young compared to mammalian ones; they provide an excellent opportunity to study the early stages of differentiation of X/Y chromosomes and sex chromosome systems (Charlesworth 2002, 2015; Ming et al 2011)
It is believed that the mechanisms of sex chromosome evolution are similar in plants and animals, but issues concerning plant sex chromosomes such as the rate and extent of addition and attrition of genetic material, localization, structure and function of sexdetermining regions, and the occurrence of dosage compensation mechanisms have not been well recognized
The North Carolina (NC) race shows the multiple sex chromosome system (XX/XY1Y2) which makes the sexes differ in the chromosome number (2n = 8 in females and 2n = 9 in males)
Summary
Sex chromosomes in flowering plants are relatively young compared to mammalian ones; they provide an excellent opportunity to study the early stages of differentiation of X/Y chromosomes and sex chromosome systems (Charlesworth 2002, 2015; Ming et al 2011). According to Smith (1963), in the NC race, sex is generally determined by X-to-autosome balance (X/A), but Y chromosomes are not neutral in gender determination because they contain genetic material necessary for expression of maleness (this system is intermediate between X/A balance observed in Rumex acetosa and the system with active Y observed in Silene latifolia). Such an intermediate sex-determining system was confirmed in the T race by Bartkowiak (1971)
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