A Comparative Analysis of Biology, Morphology, Taxonomic Identity and Genetic Diversity in Camel Melon (Citrullus lanatus) and Prickly Paddy Melon (Cucumis myriocarpus) in Australia

Camel melon (Citrullus lanatus (Thunb.) Matsum. and Nakai) and prickly paddy melon (Cucumis myriocarpus L.) are common invasive weeds across Australia and elsewhere in the world. In a recent survey, wild melons were listed as the number one weed issue in Australian summer fallow grain crops (Llewellyn et al., 2016). However, limited information is currently available on the biology, ecology or genetics of these species in Australia. This project seeks to develop a better understanding of factors assisting their invasion, including genetic variability, seed biology and phenology. Thus enhancing the ability to identify and also develop more effective weed management strategies for these species.This study collected germplasm (seeds, melon fruits and plant material) from wild melons found in 252 locations across all Australian states as well as in several other areas around the globe. All Cucumis myriocarpus, Citrullus lanatus and Citrullus colocynthis samples obtained were sequenced at two independent genomes including a nuclear gene G3pdh and a chloroplast gene ycf6-psbM of the intergenic spacer region. Australian Citrullus lanatus and Cucumis myriocarpus populations contain negligible levels of genetic diversity, indicating single, genetically impoverished founding events, potentially derived from single source populations, with the north-western region of the Indian subcontinent a likely source for the Australian Citrullus lanatus. Australian Cucumis myriocarpus plants share the same genetic profile as that of all other invasive C. myriocarpus populations, but differ from native African plants. This indicates a shared origin for invasive populations and potentially a stepping-stone pathway of founder events, the origins of which are unclear. Moderate levels of genetic diversity are present among Australian Citrullus colocynthis, geographically sorted into eastern and western regions, suggesting two separate introductions from two different source populations, most likely from northern Africa and/or southern Europe/Turkey. All study species possess distinct genotypes and haplotypes at nuclear G3pdh and chloroplast ycf6-psbM gene regions and can be identified by sequence data. Similarly, vegetative, floral and fruit characters allow for morphological identification at all life-history stages. Molecular and morphological data indicate the infraspecific identity of Australian Citrullus lanatus and Cucumis myriocarpus as Citrullus lanatus var. citroides and Cucumis myriocarpus subsp. myriocarpus, respectively. Impoverished genetic diversity of Australian Citrullus lanatus and Cucumis myriocarpus indicates that they are likely to exhibit similar responses to control measures; however, development of effective control measures for Australian Citrullus colocynthis may present a greater challenge. The use of an integrative morpho-molecular approach resulted in the discovery that Citrullus lanatus in Australia is referable to Citrullus lanatus var. citroides and that Cucumis myriocarpus is referable to Cucumis myriocarpus subsp. myriocarpus.Field studies, seed biology studies and a thorough historic literature review on Citrullus lanatus and Cucumis myriocarpus suggest the strong influence of phenotypic plasticity, breeding system, seed dormancy, allelopathy and physiological adaptation to stressful conditions on their respective and successful establishment in Australia. This study has also provided insight on the influence of genetic diversity on invasion success in two important summer fallow weeds in Australia. Further, it has provided us with information on the factors likely to support invasions as well as the potential source populations of these invasive melons in Australia.