Polyploidy arises independently of hybrid status in clonal narrow-range endemic Callistemon from East Gippsland, Victoria
Mark D. Clifton, Bryce D. Watts-Parker, Susan E. HoebeeContext
Callistemon forresterae, C. kenmorrisonii and C. nyallingensis are a group of endangered and highly clonal narrow-range endemic species from East Gippsland, Victoria, Australia. All three taxa are suspected to reproduce apomictically. Recent genomic studies have demonstrated that C. forresterae and C. kenmorrisonii were independently derived following hybridisation between the more common species C. citrinus and C. subulatus, whereas C. nyallingensis was more closely allied to C. subulatus.
Aims
This study aims to assess whether polyploidy is associated with hybridisation in the group by confirming chromosome number and c-values in each taxon with comparison with parental species.
Methods
Ploidy was assessed using seed-raised progeny of known maternal genotypes. Root tip squashes were used to determine chromosome numbers, and flow cytometric estimation of nuclear DNA content (2C values) was also used to infer ploidy.
Key results
The hybrids, C. forresterae and C. kenmorrisonii, as well as their parental species C. citrinus and C. subulatus, were confirmed to be diploid (2n = 22), whereas C. nyallingensis was found to be an autotetraploid (2n = 44). DNA content was significantly and positively correlated with observed chromosome numbers, confirming that, for Callistemon, flow cytometry can be used to determine ploidy.
Conclusions
Our results suggest that hybridisation is not a causal factor in the development of polyploidy in the East Gippsland Callistemon complex. Autopolyploidy remains the only form of polyploidy recorded in Callistemon.
Implications
If apomixis is confirmed in all taxa within the complex, then both hybridisation and polyploidy may facilitate its development. These findings have wider reaching implications, given the synonymisation of Callistemon with Melaleuca.