Genetic and cytological analyses reveal the recombination landscape of a partially differentiated plant sex chromosome in kiwifruit.

BACKGROUND: Angiosperm sex chromosomes, where present, are generally recently evolved. The key step in initiating the development of sex chromosomes from autosomes is the establishment of a sex-determining locus within a region of non-recombination. To better understand early sex chromosome evolution, it is important to determine the process by which recombination is suppressed around the sex determining genes. We have used the dioecious angiosperm kiwifruit Actinidia chinensis var. chinensis, which has an active-Y sex chromosome system, to study recombination rates around the sex locus, to better understand key events in the development of sex chromosomes. RESULTS: We have confirmed the sex-determining region (SDR) in A. chinensis var. chinensis, using a combination of high density genetic mapping and fluorescent in situ hybridisation (FISH) of Bacterial Artificial Chromosomes (BACs) linked to the sex markers onto pachytene chromosomes. The SDR is a subtelomeric non-recombining region adjacent to the nucleolar organiser region (NOR). A region of restricted recombination of around 6 Mbp in size in both male and female maps spans the SDR and covers around a third of chromosome 25. CONCLUSIONS: As recombination is suppressed over a similar region between X chromosomes and between and X and Y chromosomes, we propose that recombination is suppressed in this region because of the proximity of the NOR and the centromere, with both the NOR and centromere suppressing recombination, and this predates suppressed recombination due to differences between X and Y chromosomes. Such regions of suppressed recombination in the genome provide an opportunity for the evolution of sex chromosomes, if a sex-determining locus develops there or translocates into this region.

Strategies for isolating mutants in Hieracium with dysfunctional apomixis.

Species in the genus Hieracium subgenus Pilosella form asexual seed by the mechanism of apospory. We have reported previously that the capacity to be apomictic is inherited in these plants as a dominant trait at a single locus. The level of expression, however, is under the influence of a number of unlinked modifier loci. Several strategies are now being employed to identify and test sequences from these plants that may be involved in the control of this trait. An enhancer trap element, based on the Activator/Dissociator (Ac/Ds) transposons of maize and the ß glucuronidase reporter sequence, has been introduced into Hieracium. Introduced copies have been demonstrated to transpose in response to the co-introduction of a stabilized copy of Ac-transposase. Mutants are also being isolated following gamma irradiation of seed, and non-targeted T-DNA-mediated mutagenesis. A total of 69 stable mutants were isolated following the gamma irradiation of asexual seed. These are currently undergoing histological analysis. A further two mutants, isolated from amongst the products of Agrobacterium-mediated transformation experiments, have lost apomixis yet retain sexuality. In one mutant, the loss of apomixis appears to be associated with a failure to form aposporous initials. Although asexual seed formation has been lost in this plant, no compensatory increase in sexuality was seen. Conversely, in the second mutant, aposporous initials were seen to begin differentiation but not to divide. Unlike the first mutant, the meiotic apparatus in this plant continued to develop normally and compensating sexuality was observed.