A project undertaken at LaTrobe University, and supervised by Dr Susan Hoebee
In some plants, seed can be produced without fertilisation via a process known as apomixis. These seed are essentially clones of the seed bearing plant. In the Australian bottlebrushes (genus Callistemon) apomixis was first suggested in 1957 and has since received no further attention. It is unknown how frequently apomixis occurs in the genus and which of many developmental pathways may be involved. However, the combination of hybridisation (breeding between species) and polyploidy (more than two complete sets of chromosomes in an individual or cell) is often associated with asexual seed production. There have been recent efforts to monitor four highly localised FFG listed East Gippsland callistemons following the 2019/2020 Australian Black Summer Bushfires: C. kenmorrisonii (the Betka Bottlebrush, critically endangered), C. forresterae (Forrester’s Bottlebrush, endangered), C. genofluvialis (the Genoa River Bottlebrush, critically endangered) and C. nyallingensis (the Boggy Creek Bottlebrush, critically endangered). However, being remote and difficult to access, there is little is known about these species aside from early descriptions, and one recent study involving the population genomics of the Betka Bottlebrush. The genomic study lent support to some anecdotal suggestions that this species may be of hybrid origin. It also raised questions about whether apomixis may be the species’ mode of reproduction because the results showed extensive clonality, a lack of diversity in the adult stands, and the production of seed sharing parental genotypes. Whether polyploidy is also part of a pathway leading to apomixis in Callistemon is unknown, although polyploidy has been reported within the genus. Thus, the objectives of this project are to investigate whether hybridisation, polyploidy and apomixis are key steps in the evolutionary history of the four East Gippsland bottlebrushes. We postulate that, if these processes do occur, the survival of these species in situ may be in a delicate balance between hybridisation generating new diversity on evolutionary time scales (only if there is occasional sexual seed production), apomixis-derived clonality maintaining individual genotypes (only until regeneration is no longer possible), and large-scale environmental upheavals, such as fires and floods (catastrophic events predicted to increase in frequency and intensity). Consequently, these localised species may carry an exceptional extinction risk. This study is an extension of a broader genomics project seeking to aid and inform conservation efforts for the East Gippsland bottlebrushes through population monitoring, quantifying genotypic diversity and inbreeding, and resolving whether each of the species represent independently derived hybrid swarms involving mating between the more common C. citrinus and C. subulatus. The objectives of the study will be met using comparative genomic analyses to assess diversity and hybridisation, via chromosome preparations and flow cytometric analysis of ploidy, and via genotypic analyses of progeny, embryological histology, and flow cytometric analyses of reproductive modes to assess apomixis. It will be valuable for furthering scientific understanding of the establishment of apomixis in the Australian bottlebrushes (Callistemon), and other genera in the plant family Myrtaceae given recent suggestions that apomixis may also occur in the tea-trees (genus Leptospermum). Additionally, the investigations will also add to the current body of knowledge regarding hybridisation and polyploidy as possible causal links to apomixis.
