Abstract
BackgroundThe endemic Hawaiian mints represent a major island radiation that likely originated from hybridization between two North American polyploid lineages. In contrast with the extensive morphological and ecological diversity among taxa, ribosomal DNA sequence variation has been found to be remarkably low. In the past few years, expressed sequence tag (EST) projects on plant species have generated a vast amount of publicly available sequence data that can be mined for simple sequence repeats (SSRs). However, these EST projects have largely focused on crop or otherwise economically important plants, and so far only few studies have been published on the use of intragenic SSRs in natural plant populations. We constructed an EST library from developing fleshy nutlets of Stenogyne rugosa principally to identify genetic markers for the Hawaiian endemic mints.ResultsThe Stenogyne fruit EST library consisted of 628 unique transcripts derived from 942 high quality ESTs, with 68% of unigenes matching Arabidopsis genes. Relative frequencies of Gene Ontology functional categories were broadly representative of the Arabidopsis proteome. Many unigenes were identified as putative homologs of genes that are active during plant reproductive development. A comparison between unigenes from Stenogyne and tomato (both asterid angiosperms) revealed many homologs that may be relevant for fruit development. Among the 628 unigenes, a total of 44 potentially useful microsatellite loci were predicted. Several of these were successfully tested for cross-transferability to other Hawaiian mint species, and at least five of these demonstrated interesting patterns of polymorphism across a large sample of Hawaiian mints as well as close North American relatives in the genus Stachys.ConclusionAnalysis of this relatively small EST library illustrated a broad GO functional representation. Many unigenes could be annotated to involvement in reproductive development. Furthermore, first tests of microsatellite primer pairs have proven promising for the use of Stenogyne rugosa EST SSRs for evolutionary and phylogeographic studies of the Hawaiian endemic mints and their close relatives. Given that allelic repeat length variation in developmental genes of other organisms has been linked with morphological evolution, these SSRs may also prove useful for analyses of phenotypic differences among Hawaiian mints.
Highlights
The endemic Hawaiian mints represent a major island radiation that likely originated from hybridization between two North American polyploid lineages
The expressed sequence tag (EST) data is available through the Plant Genome Network (PGN) website [14] as well as GenBank
Reproductive developmental genes Some of the unigenes from our EST library could be identified as putative homologs of Arabidopsis genes that are active during plant reproductive development (Table 2)
Summary
The endemic Hawaiian mints represent a major island radiation that likely originated from hybridization between two North American polyploid lineages. In the past few years, expressed sequence tag (EST) projects on plant species have generated a vast amount of publicly available sequence data that can be mined for simple sequence repeats (SSRs). Based on sequence variation in the rapidly-evolving nuclear ribosomal 5S non-transcribed spacer (5S-NTS), the Hawaiian mints are most closely related to temperate North American Stachys, whereas with chloroplast (cpDNA) sequence data, they group with a largely Mesoamerican Stachys lineage (see Fig. 1). This phylogenetic incongruence between nuclear and organellar DNA data probably indicates a reticulate ancestry for the Hawaiian mints. Their closest relatives appear to be bird-pollinated Stachys chamissonis and insect-pollinated S. quercetorum
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