Evolutionary history of the Afro-Madagascan Ixora species (Rubiaceae): species diversification and distribution of key morphological traits inferred from dated molecular phylogenetic trees.

BACKGROUND AND AIMS: Previous work on the pantropical genus Ixora has revealed an Afro-Madagascan clade, but as yet no study has focused in detail on the evolutionary history and morphological trends in this group. Here the evolutionary history of Afro-Madagascan Ixora spp. (a clade of approx. 80 taxa) is investigated and the phylogenetic trees compared with several key morphological traits in taxa occurring in Madagascar. METHODS: Phylogenetic relationships of Afro-Madagascan Ixora are assessed using sequence data from four plastid regions (petD, rps16, rpoB-trnC and trnL-trnF) and nuclear ribosomal external transcribed spacer (ETS) and internal transcribed spacer (ITS) regions. The phylogenetic distribution of key morphological characters is assessed. Bayesian inference (implemented in BEAST) is used to estimate the temporal origin of Ixora based on fossil evidence. KEY RESULTS: Two separate lineages of Madagascan taxa are recovered, one of which is nested in a group of East African taxa. Divergence in Ixora is estimated to have commenced during the mid Miocene, with extensive cladogenesis occurring in the Afro-Madagascan clade during the Pliocene onwards. CONCLUSIONS: Both lineages of Madagascan Ixora exhibit morphological innovations that are rare throughout the rest of the genus, including a trend towards pauciflorous inflorescences and a trend towards extreme corolla tube length, suggesting that the same ecological and selective pressures are acting upon taxa from both Madagascan lineages. Novel ecological opportunities resulting from climate-induced habitat fragmentation and corolla tube length diversification are likely to have facilitated species radiation on Madagascar.

Floral ontogeny of the Afro-Madagascan genus Mitrasacmopsis with comments on the development of superior ovaries in Rubiaceae.

BACKGROUND AND AIMS: Members of Rubiaceae are generally characterized by an inferior ovary. However, Mitrasacmopsis is cited in the literature as having a semi-inferior to superior ovary. It has previously been hypothesized that the gynoecial development of Rubiaceae with semi-inferior to superior ovaries takes place in the same way as in Gaertnera, one of the most commonly cited rubiaceous genera with a superior ovary. To test this hypothesis, a floral ontogenetic study of Mitrasacmopsis was carried out with special attention paid to the gynoecial development. METHODS: Floral ontogeny and anatomy of Mitrasacmopsis were examined using scanning electron and light microscopy. KEY RESULTS: At an early developmental stage, a concavity becomes visible in the centre of the floral apex simultaneously with the perianth initiation. A ring primordium surrounding this concavity expands vertically forming the corolla tube (early sympetaly). Stamen primordia develop inside the corolla. From the bicarpellate gynoecium only two carpel tips are visible because the ovary is formed by a gynoecial hypanthium. In the basal part of each carpel, a placenta primordium is initiated. Two septa divide the ovary into two locules. In each locule, the placenta becomes mushroom shaped and distinctly stalked. Eventually, the inferior ovary of Mitrasacmopsis develops into a beaked capsule. Only very late in the fruiting stage, the continuously expanding ovary is raised above the insertion point of the persistent calyx, changing the ovary position of Mitrasacmopsis from basically inferior to secondarily semi-inferior. CONCLUSIONS: Mitrasacmopsis follows an epigynous pattern of floral development. However, the presence of a prominent beak in the fruiting stage gives the whole ovary a semi-inferior appearance. This kind of secondarily semi-inferior ovary is shown to be different from the secondarily superior ovary observed in Gaertnera.

Pollen morphological survey of Pentas (Rubiaceae-Rubioideae) and its closest allies.

Pollen descriptions, based on LM and SEM observations, are provided for the Pentas complex, an African alliance within the Hedyotideae-Spermacoceae alliance, which includes the genera Pentas, Otomeria, Batopedina, Parapentas, and Chamaepentas. The close relationship among these genera is confirmed by pollen morphological data. The observed variation in pollen character states agrees well with the existing genera and subgenera. The combination of small spheroidal pollen grains with narrow, acute endocolpi, and a small apocolpium index distinguishes Batopedina from Otomeria and Pentas. Parapentas differs from Batopedina in having broad endocolpi with vague ends.Two of the three subgenera of Otomeria, Neotomeria and Volubilis, are supported by pollen data. Pollen of the subgenus Otomeria, however, is less differentiated and is similar to pollen of Pentas subgenus Pentas. The exact relationship between the genera Otomeria and Pentas remains unclear, and molecular data are needed to interpret the observed morphological variation. Chamaepentas has pollen that is nearly identical to that of the Pentas subgenus Chamaepentadoides and of the subgenus Megapentas, i.e. large spheroidal pollen grains with three, medium-length colpi and endocolpi with acute ends. Pollen morphology provides additional support for a close affinity between Pentas subgenus Phyllopentas and Pentas subgenus Vignaldiopsis.