Phylogenetics of Cucumis ( Cucurbitaceae ) as understood in 2008

2007 saw the publication of two molecular phylogenies of Cucumis that relied on combined chloroplast and nuclear gene regions, and broad species sampling. One of these studies also addressed the position of Cucumis in the Cucurbitaceae family tree. It was found that the genera Cucumella, Dicaelospermum, Mukia, Myrmecosicyos, and Oreosyce are embedded within Cucumis and that the closest relative of Cucumis is Muellerargia, a genus with one species in Australia and Indonesia, the other in Madagascar. Cucumber and its sister species, C. hystrix, are nested among Australian, Malesian, and Western Indian species that had traditionally been placed in Mukia or Dicaelospermum. Cucumis melo is sister to this Australian/Asian clade, rather than being close to African species as previously thought. Molecular clock dating suggests Eocene ages for the deepest divergences within Cucumis. INTRODUCTION Knowing the closest relatives and natural composition of the genus Cucumis L. is important because of ongoing efforts by plant breeders worldwide to improve melon (C. melo) and cucumber (C. sativus) with traits from wild relatives. The year 2007 saw great progress on both fronts. First, a molecular phylogeny for the entire family (Kocyan et al. 2007) placed the genus Cucumis in a solid context based on a sample of 21% of the family’s c. 800 species and 95% of its 130 genera (following the classification of Jeffrey 2005; a new classification of the entire family will appear in the Families and Genera of Flowering Plants series published by Springer; Schaefer and Renner, submitted). Following the discovery by Kocyan et al. that Cucumis as traditionally circumscribed (Kirkbride 1993) is highly unnatural, two molecular phylogenetic studies re-investigated species relationships in a much more broadly circumscribed Cucumis (Renner et al. 2007; Ghebretinsae et al. 2007a). While largely agreeing with each other, these studies found different placements for C. melo. Here we summarize what is now known about phylogenetic relationships in Cucumis, discuss the points of disagreement between the two Cucumis phylogenies, and add new findings made since the publication of Renner et al. (2007). Cucurbitaceae 2008, Proceedings of the IX EUCARPIA meeting on genetics and breeding of Cucurbitaceae (Pitrat M, ed), INRA, Avignon (France), May 21-24, 2008