The curious chromosomes of a curious fruit

Incipient sex chromosomes have been found in New Zealand's eponymous export, the kiwifruit. Researchers writing in the open access journal BMC Genomics have mapped the kiwifruit genome and pin-pointed the sex-determining locus. It has previously been suggested that, among the kiwifruit plant's small (<1um) chromosomes, lie a pair of X/Y-like chromosomes that result in its dioecism. Now a team led by Lena Fraser and colleagues at the New Zealand Institute for Plant and Food Research has confirmed this suggestion by mapping the genome of the golden kiwifruit. The genome has been mapped with 644 microsatellite markers from three genetic libraries (two from the New Zealand Institute for Plant and Food Research and one from the University of Udine, Italy). These showed 29 linkage groups, represented expression of 587 genes, and revealed that sex-linked sequence characterised amplified region (SCAR) markers and the flower sex phenotype were mapping to a subtelomeric region that bears the hallmarks of an early sex-determining locus.

The mapping of this sex-determining locus to a subtelomeric region fits with previous published work on chromosome pairing and also the authors own observations. Whilst studying kiwifruit karyotypes, the research team observed that in the pollen mother cells undergoing meiosis one of the 29 pairs of chromosomes did not pair tightly in a region close to one end. An absence of pairing means that the male-specific region on the Y is inherited as a unit, maintaining sexual dimorphism. Based on the genetic structure they have now defined for this non-recombining sex-determining region, the authors suggest that at least two linked genes on the putative Y chromosome are responsible for dioecy: one suppressing pistil formation and one for pollen development.

Of more than 60 species of Actinidia (kiwifruit), only two have been widely cultivated so far, and there is potential for breeding new varieties. All Actinida species are dioecious, and the authors say they are likely to have similar sex-determining regions. The authors work in producing female, male and consensus genetic linkage maps for the golden kiwifruit, A. chinensis and identifying the sex-determining region may provide the key to fully exploiting this relatively recent cultivar.

According to Fraser, 'The gene-rich map we have constructed will be a valuable resource for quantitative trait loci analyses to identify markers related to traits of importance in breeding new and novel kiwifruit for the markets of the world.'

Source: BioMed Central