Refugia, dispersal and divergence in a forest archipelago: a study of Streptocarpus in eastern South Africa

We describe a scenario of plant speciation across a relict forest archipelago in South Africa involving Pleistocene habitat expansion-contraction cycles, dispersal and adaptation to lower temperatures. This is the first population level study using molecular data in South African forests and has significant implications for conservation efforts in this area. Populations of the mesophytic forest floor herbs Streptocarpus primulifolius sensu lato and Streptocarpus rexii were sampled throughout their range in the naturally fragmented forests of eastern South Africa in order to investigate population genetic and phylogenetic patterns within the species complex, using nuclear microsatellites, nuclear ribosomal ITS (internal transcribed spacer) sequences and chloroplast genome sequences. S. primulifolius harbours high levels of genetic diversity at both the nuclear (mean HE = 0.50) and the chloroplast level (each population fixed for a unique haplotype). This is consistent with populations of these coastal species being Pleistocene relicts. In contrast, populations of S. rexii in cooler habitats at higher altitudes and lower latitudes harbour little or no nuclear genetic diversity (mean HE = 0.09) and most share a common chloroplast haplotype. The split of S. rexii from populations intermediate between the two species (S. cf. primulifolius) occurred between 0 and 0.44 million years ago according to the calibrated ITS phylogeny of the taxa. The low genetic diversity and homogeneity of S. rexii is congruent with this species having reached its current range during the Holocene. We found no evidence of monophyly of any of the taxa in this study, which we consider a consequence of recent evolution in a fragmented habitat.     

Developmental analyses of the phyllomorph formation in the rosulate species Streptocarpus rexii (Gesneriaceae)

Although some species of Streptocarpus (Gesneriaceae) do not possess a layered shoot apical meristem (SAM), but three individual meristems, the basal meristem (BM), the petiolode meristem (PM) and the groove meristem (GM) on the petiolode from which additional phyllomorphs are formed. To gain insights into the processes involved, we examined the development of seedlings from germination to the formation of the primary phyllomorph in S. rexii, a rosulate species. Our specific focus was to examine the relationship between the functional activity of the GM and meristematic activity, which was assessed by a combined analysis of toluidine blue staining of histological sections and the incorporation of BrdU into meristematic tissues. The results were integrated into 3-D graphics, which suggests a complex spatial and temporal interaction within the GM. The significance of our observations is discussed and compared to the SAM observed in most other angiosperms.     

EST and random genomic nuclear microsatellite markers for Streptocarpus

Microsatellite markers have been developed from standard enriched genomic libraries and a cDNA library for the genus Streptocarpus. Out of 15 loci derived from ESTs (expressed sequence tags), four gave working primer pairs, with expected heterozygosities (H_E) ranging from 0.42 to 0.86. Out of 89 genomic library derived loci, 6 gave working primer pairs, with H_E ranging from 0.63 to 0.93.     

Developmental morphology of one-leaf plant Monophyllaea singularis (Gesneriaceae)

 Developmental morphology is described of the one-leaf plant Monophyllaea singularis which possesses a huge macrocotyledon, a long petiolode below it, and many small inflorescences scattered along the petiolode and midrib. Cell proliferation and basipetal differentiation occur in both cotyledons after water imbibition and germination. The basal meristem forms from a group of small, least differentiated cells at the base of a future macrocotyledon and continues blade production even at the reproductive stage. The petiolode meristem, which forms as an intercalary meristem near the base of the macrocotyledon, contributes to the elongation of the petiolode and the midrib. Although the 'groove meristem', like the groove meristem of Streptocarpus, forms between the cotyledons at the site of a shoot apical meristem, it is not involved in inflorescence production. In M. singularis, instead of the 'groove meristem', the inflorescences are initiated adventitiously from groups of cells in the dermal and subdermal layers of the petiolode and probably also of the midrib. Received October 10, 2000 Accepted August 2, 2001     

Origin and relationships of Saintpaulia (Gesneriaceae) based on ribosomal DNA internal transcribed spacer (ITS) sequences

Phylogenetic relationships of eight species of Saintpaulia H. Wendl., 19 species of Streptocarpus Lindl. (representing all major growth forms within the genus), and two outgroups (Haberlea rhodopensis Friv., Chirita spadiciformis W. T. Wang) were examined using comparative nucleotide sequences from the two internal transcribed spacers (ITS) of nuclear ribosomal DNA. The length of the ITS 1 region ranged from 228 to 249 base pairs (bp) and the ITS 2 region from 196 to 245 bp. Pairwise sequence divergence across both spacers for ingroup and outgroup species ranged from 0 to 29%. Streptocarpus is not monophyletic, and Saintpaulia is nested within Streptocarpus subgenus Streptocarpella. Streptocarpus subgenus Streptocarpus is monophyletic. The ITS sequence data demonstrate that the unifoliate Streptocarpus species form a clade, and are also characterized by a unique 47-bp deletion in ITS 2. The results strongly support the monophyly of (1) Saintpaulia, and (2) Saintpaulia plus the African members of the subgenus Streptocarpella of Streptocarpus. The data suggest the evolution of Saintpaulia from Streptocarpus subgenus Streptocarpella. The differences in flower and vegetative characters are probably due to ecological adaptation leading to a relatively rapid radiation of Saintpaulia.     

Breeding system of a plesiomorphic floral type: an investigation of small flowered Streptocarpus (Gesneriaceae) species

Six different floral morphologies can be found among the ca. 146 species of Streptocarpus occurring in Africa and Madagascar. One of these, a simple, small pouch type, was found to be plesiomorphic for the genus after mapping these floral types onto a molecular phylogeny. The breeding systems and population structure of three species possessing the plesiomorphic small floral morphology, S. micranthus, S. ibityensis and S. lanatus, have been investigated using nuclear microsatellite markers. Significant deviations from panmixia were found both at the between and within population level (S. micranthus θ = 0.708, f = 0.786; S. ibityensis, θ = 0.173, f = 0.138; S. lanatus, θ=0.539, f = 0.646). As a florally diverse genus, Streptocarpus is unusual in having a predominantly selfing, mixed mating breeding system as a plesiomorphic condition.     

Multiple Pleistocene refugia and recent diversification for Streptocarpus ionanthus (Gesneriaceae) complex: Insights from multiple molecular sources

Streptocarpus ionanthus (Gesneriaceae) is endemic to Tanzania and Kenya, distributed in Tanga, Morogoro, and Kilifi regions. The species houses nine subspecies characterized by complex morphotypes and poorly understood evolutionary relationships, and thus is an ideal model for investigating evolutionary dynamics over time. Using multiple methods, we sought to test our hypothesis that the infraspecific taxa in Str. ionanthus are slightly variable and evolving populations. We first examined the genetic diversity, population differentiation, and phylogeographic structure among the populations of Str. ionanthus using both chloroplast and nuclear markers. We then estimated the divergence time of Str. ionanthus lineages and modeled past and future distribution. Despite Str. ionanthus exhibiting bottleneck events across its range, the populations maintain relatively high genetic diversity attributed to historical population admixture or local adaptation arising from habitat heterogeneity. The phylogeographic and genetic structure revealed a high connection among the Usambara mountains populations, while molecular dating suggested most diversification of haplotypes began ~1.32–0.18 million years ago and intensified toward the present, a conclusion of recent diversification. Phylogenetic relationship of Str. ionanthus cpDNA haplotypes revealed five main lineages with unique haplotypes that could be suggestive of past isolated refugia during the Pleistocene climate shifts. According to niche modeling, the stability of suitable areas during the Last Glacial Maximum (LGM) offered protective micro-habitats that have preserved the genetic diversity of Str. ionanthus to date. In conclusion, our findings suggest a complex Str. ionanthus with slightly variable lineages or populations attributed to multiple refugia and on the verge of divergence.     

One-leaf plants in the Gesneriaceae: Natural mutants of the typical shoot system

The aerial part of seed plants is called the shoot, which is composed of stems, leaves, and axial buds. These are produced by indeterminate activity in the shoot apical meristem (SAM), whereas the morphogenesis of leaves depends on determinate activity of leaf meristems. However, one-leaf plants in the Gesneriaceae family (eudicots) do not have a typical SAM and do not produce new organs when in the vegetative phase. Instead, they have one cotyledon whose growth is indeterminate. This peculiar development is supported by the groove meristem, which corresponds to the canonical SAM, and the basal meristem, which corresponds to the typical leaf meristem. However, the former does not produce any organ and the latter is active indeterminately. Gene expression and physiological analyses have been conducted in an effort to determine the molecular nature of this peculiar organogenesis. This review summarizes the current understanding of the development of one-leaf plants to provide future perspectives in this field of research.     

Characterization of anisocotylous leaf formation in Streptocarpus wendlandii (Gesneriaceae): significance of plant growth regulators

BACKGROUND AND AIMS: Unifoliate species of Gesneriaceae are unique, as they bear only one leaf throughout their life history. The development of this leaf (termed a macrocotyledon) derived from one of two cotyledons is intriguing. The other cotyledon does not develop further and is termed a microcotyledon. This process of unequal cotyledon development is termed anisocotyly. In this study the process of macrocotyeldon formation was studied and the effects of plant hormones on the macrocotyledon development were investigated. METHODS: Streptocarpus wendlandii was chosen as the main subject material, as it was found to be suitable for experimental studies in laboratory conditions. Morphological analyses were carried out with light and scanning electron microscopy. Plant hormones were applied exogenously. KEY RESULTS: The macrocotyledon of S. wendlandii is produced through cell division activity in the basal meristem of the enlarging cotyledon. The newly developed region in the macrocotyledon displayed distinct morphological changes, including the formation of long, needle-shaped trichomes. The newly formed region was surrounded by lateral veins. No such change was observed in the microcotyledon. Furthermore, it was shown that development of anisocotyly is suppressed by the application of cytokinin, resulting in the formation of two nearly equal-sized cotyledons. Both cotyledons displayed macrocotyledon characteristics. This observation in S. wendlandii was confirmed using Monophyllaea glabra, another unifoliate species in the same family. CONCLUSIONS: It is proposed that developmental changes of the macrocotyledon have characteristics of a developmental phase-change, and cytokinins may be involved in its formation. These results are discussed in the light of current knowledge of phase-change transitions in plant vegetative development.     

EVOLUTION OF MORPHOLOGICAL NOVELTY: A PHYLOGENETIC ANALYSIS OF GROWTH PATTERNS IN STREPTOCARPUS (GESNERIACEAE)

Abstract.— Streptocarpus shows great variation in vegetative architecture. In some species a normal shoot apical meristem never forms and the entire vegetative plant body may consist of a single giant cotyledon, which may measure up to 0.75 m (the unifoliate type) or with further leaves arising from this structure (the rosulate type). A molecular phylogeny of 87 taxa (77 Streptocarpus species, seven related species, and three outgroup species) using the internal transcribed spacers and 5.8S region of nuclear ribosomal DNA suggests that Streptocarpus can be divided into two major clades. One of these broadly corresponds to the caulescent group (with conventional shoot architecture) classified as subgenus Streptocarpella, whereas the other is mainly composed of acaulescent species with unusual architecture (subgenus Streptocarpus). Some caulescent species (such as S. papangae) are anomalously placed with the acaulescent clade. Available cytological data are, however, completely congruent with the two major clades: the caulescent clade is x = 15 and the acaulescent clade (including the caulescent S. papangae) is x = 16 (or polyploid multiples of 16). The genera Linnaeopsis, Saintpaulia, and Schizoboea are nested within Streptocarpus. The sequenced region has evolved, on average, 2.44 times faster in the caulescent clade than in the acaulescent clade and this is associated with the more rapid life cycle of the caulescents. Morphological variation in plant architecture within the acaulescent clade is homoplastic and does not appear to have arisen by unique abrupt changes. Instead, rosulate and unifoliate growth forms have evolved several times, reversals have occurred, and intermediate architectures are found. An underlying developmental plasticity seems to be a characteristic of the acaulescent clade and is reflected in a great lability of form.