Abstract
Rumex hastatulus is the North American endemic dioecious plant with heteromorphic sex chromosomes. It is differentiated into two chromosomal races: Texas (T) race characterised by a simple XX/XY sex chromosome system and North Carolina (NC) race with a polymorphic XX/XY1Y2 sex chromosome system. The gross karyotype morphology in NC race resembles the derived type, but chromosomal changes that occurred during its evolution are poorly understood. Our C-banding/DAPI and fluorescence in situ hybridization (FISH) experiments demonstrated that Y chromosomes of both races are enriched in DAPI-positive sequences and that the emergence of polymorphic sex chromosome system was accompanied by the break of ancestral Y chromosome and switch in the localization of 5S rDNA, from autosomes to sex chromosomes (X and Y2). Two contrasting domains were detected within North Carolina Y chromosomes: the older, highly heterochromatinised, inherited from the original Y chromosome and the younger, euchromatic, representing translocated autosomal material. The flow-cytometric DNA estimation showed ∼3.5 % genome downsizing in the North Carolina race. Our results are in contradiction to earlier reports on the lack of heterochromatin within Y chromosomes of this species and enable unambiguous identification of autosomes involved in the autosome-heterosome translocation, providing useful chromosome landmarks for further studies on the karyotype and sex chromosome differentiation in this species.Electronic supplementary materialThe online version of this article (doi:10.1007/s10577-014-9446-4) contains supplementary material, which is available to authorized users.
Highlights
Heteromorphic sex chromosomes are known only from a limited number of dioecious plants
We demonstrate that Y chromosomes of this species are enriched with DAPI-positive sequences, and that the reduction of chromosome number in North Carolina (NC) race was accompanied by the switch in the localization of 5S rDNA from autosomes to heterosomes
Before squashing in 45 % acetic acid, fixed root tips were hydrolyzed in 1 M HCL at 60 °C for 13 min or macerated enzymatically (1 % pectinase+1 % cellulase in citric buffer, ph 4.6) at 37 °C for 30 min
Summary
Heteromorphic sex chromosomes are known only from a limited number of dioecious plants. Angiosperms are of particular interest for empirical studies of sex chromosome evolution, because they evolved separate sexes repeatedly and relatively recently (Charlesworth 2002). They developed, like animals, either simple (XX/XY) or complex (polymorphic) sex chromosome systems (Matsunaga and Kawano 2001, Vyskot and Hobza 2004, Jamilena et al 2008, Weingartner and Delph 2014). Humulus and Rumex, revealed the presence of two different sex chromosome systems—XX/XY and XX/XY1Y2 (Parker and Clark 1991, Mariotti et al 2009). In comparison to XX/XY relatives, the lower chromosome counts in XX/XY1Y2 species may support this hypothesis
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