Asymmetrical gene flow in a hybrid zone of Hawaiian Schiedea (Caryophyllaceae) species with contrasting mating systems.

Asymmetrical gene flow, which has frequently been documented in naturally occurring hybrid zones, can result from various genetic and demographic factors. Understanding these factors is important for determining the ecological conditions that permitted hybridization and the evolutionary potential inherent in hybrids. Here, we characterized morphological, nuclear, and chloroplast variation in a putative hybrid zone between Schiedea menziesii and S. salicaria, endemic Hawaiian species with contrasting breeding systems. Schiedea menziesii is hermaphroditic with moderate selfing; S. salicaria is gynodioecious and wind-pollinated, with partially selfing hermaphrodites and largely outcrossed females. We tested three hypotheses: 1) putative hybrids were derived from natural crosses between S. menziesii and S. salicaria, 2) gene flow via pollen is unidirectional from S. salicaria to S. menziesii and 3) in the hybrid zone, traits associated with wind pollination would be favored as a result of pollen-swamping by S. salicaria. Schiedea menziesii and S. salicaria have distinct morphologies and chloroplast genomes but are less differentiated at the nuclear loci. Hybrids are most similar to S. menziesii at chloroplast loci, exhibit nuclear allele frequencies in common with both parental species, and resemble S. salicaria in pollen production and pollen size, traits important to wind pollination. Additionally, unlike S. menziesii, the hybrid zone contains many females, suggesting that the nuclear gene responsible for male sterility in S. salicaria has been transferred to hybrid plants. Continued selection of nuclear genes in the hybrid zone may result in a population that resembles S. salicaria, but retains chloroplast lineage(s) of S. menziesii.

Estimating the species tree for Hawaiian Schiedea (Caryophyllaceae) from multiple loci in the presence of reticulate evolution.

Schiedea (Caryophyllaceae) is a monophyletic genus of 34 species, all endemic to the Hawaiian Islands, that arose from a single colonization, providing one of the best examples of adaptive radiation in Hawai'i. Species utilize a range of habitats and exhibit a variety of growth forms and transitions in breeding systems from hermaphroditism toward dimorphism or autogamy. Our study included the most thorough sampling to date: 2-5 individuals per species and 4 independent genetic partitions: eight plastid and three low-copy nuclear loci (9217bps), allowing a three-locus BEST species tree. Despite incomplete resolution at the tips, our results support monophyly for each extant species. Gene trees revealed several clear cases of cytonuclear incongruence, likely created by interspecific introgression. Conflict occurs at the divergence of section Alphaschiedea as well as at the tips. Ages inferred from a BEAST analysis allow an original colonization onto either Nihoa or Kauaì and inform some aspects of inter-island migrations. We suggest that several hard polytomies on the species tree are biologically realistic, signifying either nearly simultaneous speciation or historical introgressive hybridization. Based on inferred node ages that exceed expected coalescent times, we propose that undetected nuclear introgression may play a larger role than incomplete lineage sorting in sections Schiedea and Mononeura.

Phylogeographic patterns and demographic history of Schiedea globosa (Caryophyllaceae) on the Hawaiian Islands.

Geomorphological changes have been demonstrated to have had profound impacts on biodiversity, often leading to demographic expansions and contractions and allopatric divergence of taxa. We examined DNA sequence variation at two nuclear and one maternally inherited plastid locus among 10 populations of Schiedea globosa on the Hawaiian Islands to assess the primary factors shaping genetic structure, phylogeographic patterns, and the importance of geographic isolation to population divergence. Schiedea globosa has characteristics that may promote gene flow, including wind pollination and rafting of plants in ocean currents. However, we detected significant differentiation among populations on all islands except Hawaii, with the maternally inherited plastid locus having the greatest genetic structure (F(ST) = 0.81). Migration rates across all loci are less than one migrant per generation. We found evidence of growth in several populations and on the islands of Molokai and Maui, which supports population expansion associated with the formation of Maui Nui during the last glacial maximum. Similar to data for many other Hawaiian taxa, these data suggest S. globosa originated on Oahu and subsequently colonized Molokai, Maui, and Hawaii in progression. Given the high level of genetic structure, allopatric divergence will likely contribute to further divergence of populations.

The origin and number of introductions of the Hawaiian endemic Silene species (Caryophyllaceae).

The Hawaiian endemic Silene are a small group of woody or semiwoody representatives from a large, predominantly herbaceous, species-rich genus. We here investigated the origin and number of introductions of the endemic Hawaiian Silene based on phylogenetic relationships inferred from DNA sequences from both the plastid (the rps16 intron) and the nuclear (ribosomal internal transcribed sequences, ITS, and intron 23 of the RPB2 gene) genomes. Silene antirrhina, a widespread weedy American annual, is strongly supported as sister to a monophyletic group consisting of the Hawaiian Silene, indicating a single colonization event. There are no obvious morphological similarities between S. antirrhina and any of the species of Hawaiian Silene. Our results suggest an American origin for the Hawaiian endemics because that would require only a single trans-ocean dispersal. Two of the Hawaiian endemics (S. struthioloides and S. hawaiiensis) that form a subclade in the analyses have evolved woodiness after introduction to the Hawaiian Islands. Our results contribute to other recent results based on molecular phylogenetics that emphasize the American continent as a source area for the Hawaiian flora and support a striking morphological radiation and evolution of woodiness from a single introduction to the archipelago.

Reconstructing ancestral patterns of colonization and dispersal in the Hawaiian understory tree genus Psychotria (Rubiaceae): a comparison of parsimony and likelihood approaches.

Systematic and biogeographical relationships within the Hawaiian clade of the pantropical understory shrub genus Psychotria (Rubiaceae) were investigated using phylogenetic analysis of 18S-26S ribosomal DNA internal (ITS) and external (ETS) transcribed spacers. Phylogenetic analyses strongly suggest that the Hawaiian Psychotria are monophyletic and the result of a single introduction to the Hawaiian Islands. The results of phylogenetic analyses of ITS and ETS partitions alone give slightly different topologies among basal lineages of the Hawaiian clade; however, such differences are not well supported. Relationships in the section Straussia clade in particular are not well resolved because of few nucleotide changes on internal branches, suggesting extremely rapid radiation in the lineage. Parsimony and likelihood reconstructions of ancestral geographical distributions using the topologies inferred from both parsimony and likelihood analysis of combined data and using different combinations of models and branch lengths gave highly congruent results. However, for one internal node (corresponding to the majority of the "greenwelliae" clade), parsimony reconstructions were unable to distinguish between three possible island states, whereas likelihood reconstructions resulted in clear ordering of possible states, with the island of Oàhu slightly more probable than other islands under all but one model and branch length combination considered (the Jukes-Cantor-like model with branch lengths inferred under parsimony, under which conditions Maui Nui is more probable). A pattern of colonization from oldest to youngest islands was inferred from the phylogeny, using maximum parsimony and maximum likelihood. Additionally, a much higher incidence of intraisland versus interisland speciation was inferred.

Family-level relationships of Onagraceae based on chloroplast rbcL and ndhF data.

Despite intensive morphological and molecular studies of Onagraceae, relationships within the family are not fully understood. One drawback of previous analyses is limited sampling within the large tribe Onagreae. In addition, the monophyly of two species-rich genera in Onagreae, Camissonia and Oenothera, has never been adequately tested. To understand relationships within Onagraceae, test the monophyly of these two genera, and ascertain the affinities of the newly discovered genus Megacorax, we conducted parsimony and maximum likelihood analyses with rbcL and ndhF sequence data for 24 taxa representing all 17 Onagraceae genera and two outgroup Lythraceae. Results strongly support a monophyletic Onagraceae, with Ludwigia as the basal lineage and a sister-taxon relationship between Megacorax and Lopezia. Gongylocarpus is supported as sister to Epilobieae plus the rest of Onagreae, although relationships within the latter clade have limited resolution. Thus, we advocate placement of Gongylocarpus in a monogeneric tribe, Gongylocarpeae. Most relationships within Onagreae are weakly resolved, suggesting a rapid diversification of this group in western North America. Neither Camissonia nor Oenothera appears to be monophyletic; however, increased taxon sampling is needed to clarify those relationships. Morphological characters generally agree with the molecular data, providing further support for relationships.

Urticalean rosids: circumscription, rosid ancestry, and phylogenetics based on rbcL, trnL-F, and ndhF sequences.

To address the composition of the urticalean rosids, the relationships of the component families (maximally Cannabaceae, Cecropiaceae, Celtidaceae, Moraceae, Ulmaceae, and Urticaceae) and analyze evolution of morphological characters, we analyzed sequence variation for a large sampling of these families and various rosid outgroups using rbcL, trnL-F, and ndhF plastid regions. Urticalean rosids are derived out of a lineage including Barbeyaceae, Dirachmaceae, Elaeagnaceae, and Rhamnaceae, with Rosaceae less closely related; thus, they are imbedded within Rosales. Ulmaceae are the sister to all remaining families. Cannabaceae are derived out of a subclade of Celtidaceae; this expanded family should be called Cannabaceae. Cecropiaceae are derived within Urticaceae and are polyphyletic with Poikilospermum derived elsewhere within Urticaceae; this expanded family should be called Urticaceae. Monophyletic Moraceae are sister to this expanded Urticaceae. Support for these relationships comes from a number of morphological characters (floral sexuality, presence or absence of hypanthium, stamen type and dehiscence, pollen pore number, ovule position, and embryo alignment) and chromosome numbers. Most fruit types, in terms of ecological dispersal, are derived independently multiple times and are strongly correlated with habitat.