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https://www.ias.ac.in/article/fulltext/jgen/093/01/0241-0277

dioecy-mediated species evolution; plant unisexuality; sex-chromosome evolution; sex-ratio variation; XY–WZ transition; Y-chromosome degeneration.

Dioecy (separate male and female individuals) ensures outcrossing and is more prevalent in animals than in plants. Although it is common in bryophytes and gymnosperms, only 5% of angiosperms are dioecious. In dioecious higher plants, flowers borne on male and female individuals are, respectively deficient in functional gynoecium and androecium. Dioecy is inherited via three sex chromosome systems: XX/XY, XX/X0 and WZ/ZZ, such that XX or WZ is female and XY, X0 or ZZ are males. The XX/XY system generates the rarer XX/X0 andWZ/ZZ systems. An autosome pair begets XY chromosomes. A recessive loss-of-androecium mutation (ana) creates X chromosome and a dominant gynoecium-suppressing (GYS) mutation creates Y chromosome. The ana/ANA and gys/GYS loci are in the sex-determining region (SDR) of the XY pair. Accumulation of inversions, deleterious mutations and repeat elements, especially transposons, in the SDR of Y suppresses recombination between X and Y in SDR, making Y labile and increasingly degenerate and heteromorphic from X. Continued recombination between X and Y in their pseudoautosomal region located at the ends of chromosomal arms allows survival of the degenerated Y and of the species. Dioecy is presumably a component of the evolutionary cycle for the origin of new species. Inbred hermaphrodite species assume dioecy. Later they suffer degenerate-Y-led population regression. Cross-hybridization between such extinguishing species and heterologous species, followed by genome duplication of segregants from hybrids, give rise to new species.

Sushil Kumar^{1 2} Renu Kumari^{1 2} Vishakha Sharma¹

1. SKA Institution of Research, Education and Development (SKAIRED), 4/11 SarvPriya Vihar, New Delhi 110 016, India
2. National Institute of Plant Genome Research, Aruna Asaf Ali Marg, New Delhi 110 067, India

Published

April 2014

Manuscript received

16 June 2013

Manuscript revised

31 August 2013

Accepted

5 September 2013

Early published

15 April 2014