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 β 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.