Nuclear Ribosomal RNA Sequences for Inferring Tribal Relationships Within Onagraceae

Abstract

Relationships among tribes within the evening-primrose family, Onagraceae, were inferred from 18S and 26S ribosomal RNA sequences. Of 1819 nucleotide positions sequenced, 17 were phylogenetically informative. With Lythrum (Lythraceae) as the outgroup, one most parsimonious cladogram was derived (length = 28; consistency index = 0.75) using computer-assisted phylogeny inference software. The cladogram supported Ludwigia as the sister group to Onagraceae and provided strong evidence for the monophyly of Oenothera-Epilobium and Circaea-Fuchsia. The placements of Hauya and Lopezia were not well supported by the data as indicated by examining trees which were one step longer than the most parsimonious tree. Onagraceae is the best known and most thoroughly studied family in the order Myrtales (Raven 1989) and has been of evolutionary and genetic interest since deVries' classic analyses of non-Mendelian inheritance in mutants of Oenothera in 1886 (Cleland 1972). It is distinct from the other families in Myrtales by virtue of the following synapomorphic character-states: epigynous flowers (Eyde 1981), pollen with viscin threads (Skvarla et al. 1978), paracrystalline beaded ektexine (Skvarla et al. 1978), 4-nucleate embryo-sac formation (Raven 1979; Tobe and Raven 1983), vegetative tissues abundant with calcium oxalate raphides (Metcalfe and Chalk 1950; Carlquist 1975) and divided sporogenous (anther) tissue (Tobe and Raven 1986). The family consists of seven tribes, 16 genera and approximately 650 species (Raven 1989). Detailed information is available for much of this family for floral and vegetative anatomy and morphology (Carlquist 1977; Eyde 1982), breeding systems and pollinators (Raven 1979), embryology (Tobe and Raven 1983), pollen morphology (Patel et al. 1984), flavonoids (Averett and Raven 1984), and chromosome features (Kurabayashi et al. 1962). Although Onagraceae is well-characterized at many levels, tribal concepts and relationships within the family remain uncertain. The genera within Onagraceae are highly distinctive, suggesting that the lineages within the family diverged very early in its history (Raven 1989). If this is true, it is not surprising that evolutionary connections between tribes have proved difficult to resolve. Current classification places Ludwigia (tribe Jussiaeeae) as the sister group to all other Onagraceae, based on evidence from floral anatomy (Eyde 1977, 1979, 1981, 1982), cytology (Raven and Tai 1979) and amino acid sequence differences for the N-terminal region of the small subunit of ribulose bisphosphate carboxylase (RuBisCo; Martin and Dowd 1986). Fuchsieae, Circaeeae, Lopezieae, and Hauyeae are considered to be closely related because they are the least specialized members of the family with respect to floral and vegetative anatomy and chromosome number (Dahlgren and Thorne 1984; Raven 1989). At one time, Hauya was considered to be the basal genus of Onagreae (Tobe and Raven 1985; Raven 1989) based on similarities of pollen (Skvarla et al. 1975), chromosomes (Kurabayashi et al. 1962) and floral morphology (Raven 1979). It currently is placed in a separate tribe (Hauyeae) because the genera possess a number of features, such as presence of stipules, lack of interxylary phloem and a generalized leaf anatomy (Tobe and Raven 1985, 1986), that distinguish them from members of Onagreae. Tobe and Raven (1986) suggest a possible relationship between Onagreae (Clarkia) and Epilobieae, because both are highly specialized groups having unique four-lobed stigmas with the lobes in commissural position and dry, papillate stigma surfaces (Raven 1979; Eyde 1982). Along with Lopezia, they are the only groups in Onagraceae to display marked protandry (Raven 1979). In this study, nuclear ribosomal RNA (rRNA)