Abstract
BackgroundThe southern African Oxalis radiation is extremely morphologically variable. Despite recent progress in the phylogenetics of the genus, there are few morphological synapomorphies supporting DNA-based clades. Leaflet anatomy can provide an understudied and potentially valuable source of information on the evolutionary history and systematics of this lineage. Fifty-nine leaflet anatomical traits of 109 southern African Oxalis species were assessed in search of phylogenetically significant characters that delineate clades.ResultsA combination of 6 leaflet anatomical traits (stomatal position, adaxial epidermal cells, abaxial epidermal cells, mesophyll, sheath around vascular tissue, degree of leaflet conduplication) clearly support various clades defined by previous DNA-based phylogenetic work. Other, mostly continuous leaflet anatomical traits were highly variable and showed less phylogenetic pattern.ConclusionsMajor and unexpected findings include the transition from ancestral hypostomatic leaflets to adaxially-located stomata in the vast majority of southern African Oxalis, the loss of semi-swollen AB epidermal cells and the gain of swollen adaxial and abaxial epidermal cells in selected clades, and multiple changes from ancestral bifacial mesophyll to isobilateral or homogenous mesophyll types. The information gathered in this study will aid in the taxonomic revision of this speciose member of the Greater Cape Floristic Region and provide a basis for future hypotheses regarding its radiation.Electronic supplementary materialThe online version of this article (doi:10.1186/s12862-016-0792-z) contains supplementary material, which is available to authorized users.
Highlights
The southern African Oxalis radiation is extremely morphologically variable
The All rates different (ARD) model fit this subset of trees substantially better than the SYM model, we argue that the chances of homogenous mesophyll being ancestral in SA Oxalis are unlikely, given the distribution of bifacial mesophyll in other southern African Oxalis taxa and bifacial mesophyll in some studied outgroup taxa
The findings of this study showed that a combination of six leaflet anatomical traits clearly support various southern African Oxalis clades defined by previous DNA-based phylogenetic work [19]
Summary
The southern African Oxalis radiation is extremely morphologically variable. Despite recent progress in the phylogenetics of the genus, there are few morphological synapomorphies supporting DNA-based clades. Fifty-nine leaflet anatomical traits of 109 southern African Oxalis species were assessed in search of phylogenetically significant characters that delineate clades. Much of our current understanding of angiosperm relationships has been achieved through analysis of DNA data, there is still great potential to find phylogenetically informative morphological characters in many angiosperm groups, in groups that have been morphologically poorly-studied. Oxalis L. is by far the largest genus in the Oxalidaceae, including more than 500 species [8, 9] This morphologically variable genus includes shrubs, stem succulents, herbs, annuals and geophytes, with a cosmopolitan distribution [5, 10,11,12]. All members of the southern African Oxalis lineage are geophytic [18], in contrast to approximately 31 % of the species from the South American diversity centre [9]
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