Diversity and Evolution of Seedling Baupläne in Perlargonium (Geraniaceae)

Abstract

The genus Pelargonium (Geraniaceae) exhibits tremendous variation in growth form. We apply a broadly defined concept of Bauplan in our study of growth form, architectural and anatomical features of early seedling development in the type subgenus. We analyze variation in these features within a phylogenetic framework based on sequence comparisons of internal transcribed spacer regions (ITS) of nuclear ribosomal DNA.Preliminary ITS sequence comparisons show strong support for two major clades. One major clade contains two subgroups, one consisting of three previously recognized sections of more or less woody shrubs and subshrubs (sections Pelargonium, Glaucophyllum and Campylia), and a second clade of five sections consisting of geophytes and branched stem-succulents (sections Hoarea, Ugularia, Polyactium, Cortusina and Otidia). The shrubby subgroup is characterized by relatively high levels of variation in architectural determinants such as internode elongation and branching. All members of this subgroup exhibit similar patterns of tissue production by the vascular cambium wherein secondary xylem consists predominantly of lignified tracheary elements and fibers. The stem-succulent clade shows less variation in some architectural determinants such as internode length and branching. The location of the initial onset of axis thickening is variable among species. In all sections except Ligularia, axis thickening is correlated with the onset of cambial activity wherein both secondary xylein and phloem are composed of few conducting cells scattered among abundant storage parenchyma. Section Ligularia is characterized by cambial activity similar to that in the shrubby clade; epicotylar thickening in this section arises through expansion and division of cortical cells.The second major clade within the type subgenus contains the monotypic section Isopetalum, and sections Peristera and Reniformia. Members of these sections range from shrubs that are woody at the base to herbaceous scramblers. Species examined exhibit variation in branching but limited internodal elongation. These seedlings were thin stemmed and showed relatively limited activity of a vascular cambium, which produced secondary xylem consisting of lignified tracheary elements and fibers. The occurrence of a vascular cambium producing largely lignified tracheary elements and fibers in all members of the subgenus except the stem-succulent clade suggests that this type of secondary xylem is basal within the subgenus, and that section Ligularia has either retained or reevolved the basal state.

Shifts in Climate Foster Exceptional Opportunities for Species Radiation: The Case of South African Geraniums

“…This indicates that much of the evolution of the climate niche in the genus has occurred independent of the recently created winter precipitation gradients and that pre-adaptation to summer aridity might have played an important role in diversification [14]. Diversification in Pelargonium has therefore mainly occurred along the increased summer aridity gradients, agreeing with extended diversification in drought adapted growth forms, leaf attributes and phenologies in the genus [22], [45]. The small chromosome clade A exhibits a much wider variation in morphological adaptations to cope with drought when compared to the large chromosome clade lineages [22].…”

Developmental patterns in anatomy are shared among separate evolutionary origins of stem succulent and storage root‐bearing growth habits in Adenia (Passifloraceae)

“…The evolution of vascular strands such as cortical bundles (strands in the cortex), medullary bundles (strands in the pith), or strands in the axial system occurred in a variety of succulent groups with wide stems ( Olson, 2003 ;Mauseth, 1993Mauseth, , 2004Mauseth and Sajeva, 1992), but they were not observed in relatively thin seedlings of succulent Pelargonium (Geraniaceae; Jones and Price, 1996 ). In each evolutionary origination of succulent stems and storage roots, is the coordinated evolution of VSs a consequence?…”

Geraniaceae

“…The differential development of the hypocotyl, internode elongation and branching system of various life forms in Pelargonium have been studied by Jones and Price (1996). Hypseocharis is an acaulescent or short-stemmed hemicryptophyte.…”

“…Cortusina primary and secondary tissues take part in its formation (Jones and Price 1996). Hoarea, a phellogen originating in the outermost cortical parenchyma produces numerous periderm layers.…”