The biology of Cassytha pubescens An Australian native hemiparasitic vine

Abstract

This project improves our general understanding of the fundamental biology of one of our most interesting native plants, Cassytha pubescens. The implications of this research are far-reaching across many fields and have answered many diverse questions. To begin with, I collated all known dispersal vectors for all Cassytha species in a literature review, before examining scats of Isoodon obesulus and determining it may act as a dispersal vector for C. pubescens seeds. As the historical distribution of both species overlap significantly, it is likely that I. obesulus has played an important role in seed movement for the parasite over time. C. pubescens forms patches seemingly growing clonally on different hosts, so I used genetic markers (SNPs) to understand the relatedness of individuals within and between sites at varying scales and to determine the relative importance of clonal reproduction for this species. Interestingly, my results show a high prevalence of clonal reproduction within sites, and diverging populations between national parks. This indicates seed dispersal may be limited, and that vegetative reproduction plays an important role in allowing populations to persist over long periods of time. Fire is thought to stimulate germination in C. pubescens, based on laboratory testing. The field observations presented here are the first to confirm a strong fire response in this species. Importantly, significant time-delays were found between the burn and seedling emergence. Patterns of emergence in relation to the location of previous host plants, as well as observations of seed traps within the immediate landscape, also imply limited dispersal for this species. As seedlings emerged in high densities, I examined the effect of high seedling density at emergence, and assessed whether host species had any effect on their survivorship. Significant differences in survivorship were found between seedlings emerging at low and high density, indicating there is competition for resources between siblings. I used a glasshouse experiment to examine the effect of host choice and density on establishment of seedlings. Contrary to expectations, greater host choice (and a correspondingly greater chance to find a host) did not correlate to greater establishment. It is proposed here that as seen in the field, competition plays a significant role in seedling success as greater host density increases competition and limits resource availability for emerging seedlings. I also used Rubus fruitcosus in a glasshouse experiment to test whether C. pubescens seedlings could establish on a non-native host, and examined their effect on the performance of a host species with marked modular growth. I found that the parasite established well, induced strong resource partitioning effects in R. fruticosus, and that infection has a significant effect on overall biomass and thorniness. With these results, interesting parallels are drawn between the effects of parasitism and herbivory. Finally, I present a compilation of information collected from the literature and from observations and short experiments. This includes a literature and herbarium data review to give perspective on the breadth of the host range for the parasite. Conditions required for seed germination, some results regarding seedling ecology, as well as an investigation into any potential mycorrhizal associations are also discussed.