Phylogenetic Relationships and the Evolution of Ovary Position in Saxifraga section Micranthes

Abstract

Section Micranthes of the genus Saxifraga (Saxifragaceae) comprises 67 species that are distrib- uted throughout the northern hemisphere. A previous phylogenetic analysis indicates that this section is a lineage distinct from the remainder of Saxifraga. Recent taxonomic treatments have divided section Micranthes into four subsections: Cuneifoliatae, Micranthes, Rotundifoliatae, and Stellares. To investigate the phylogenetic relationships among species in section Micranthes and to test the monophyly of each of the four subsections, we sequenced the chloroplast gene matK for 26 species of section Micranthes. The results of our parsimony analyses suggest that subsections Micranthes and Stellares are each monophyletic. The single taxon of subsec- tion Cuneifoliatae (S. calycina) for which material could be obtained appears within a clade representing sub- section Rotundifoliatae; hence Cuneifoliatae may not be distinct from Rotundifoliatae. Within section Micranthes there exists a high diversity of ovary positions, ranging from what has been described as fully superior to greater than one-half inferior. Examination of this character in light of our matK strict consensus tree indicates that the major trend in gynoecial evolution in section Micranthes has been from an ancestor with what has been termed a superior ovary towards greater inferiority. However, gynoecial evolution in subsection Mi- cranthes is complex, with several apparent reversals towards greater superiority. The genus Saxifraga, as traditionally defined, con- sists of approximately 440 species and is the largest genus in Saxifragaceae s. s. (Webb and Gomall 1989). Saxifraga is distributed throughout the north temperate and Arctic regions, with centers of di- versity in the mountains of Europe, eastern Asia, and western North America. The genus is extreme- ly diverse in morphology, ecology, habit, and cy- tology, which has led to difficulty in defining its limits. For example, Haworth (1812) divided Saxi- fraga into 16 segregate genera (see also Small and Rydberg 1905; Love 1983). However, many inves- tigators prefer a broad interpretation of Saxifraga (e.g., Engler 1872; Engler and Irmscher 1916; Gor- nall 1987; Webb and Gornall 1989). Recent phylo- genetic analyses of cpDNA restriction sites (Soltis et al. 1993), rbcL and matK sequences (Johnson and Soltis 1994, 1995; Soltis et al. 1996), and ITS se- quences (Johnson and Soltis 1998) indicate clearly that Saxifraga is polyphyletic, with species forming two distinct clades-Saxifraga s. s. and Micranthes. The former comprises most of the 15 sections tra- ditionally placed in Saxifraga, whereas the Micran- thes clade includes Saxifraga section Micranthes (67 species). The closest relatives of the Micranthes clade appear to be Saxifraga section Merkianae (two spe- cies), and the monotypic Cascadia (= S. nuttallii). Based primarily on features of the seed coat (Kaplin 1981), Webb and Gornall (1989) recognized four subsections within section Micranthes: Micran-