Ecological factors and developmental preconditions shaping the evolution of secondary woodiness in the umbellifer subfamily Apioideae

Goal: The aim of the project is to evaluate the importance of ecological factors and ontogenetic (anatomical) constraints that shape the evolution of secondary woodiness in a clade of herbaceous angiosperms: family Apiaceae subfamily Apioideae. From the ecological perspective, woodiness is a strategy of surviving cold or dry season by producing perennating buds above ground. In the anatomical categories, woodiness is characterized by an intense production of secondary xylem that may occur both in annuals and in perennials due to a ring of secondary (lateral) meristem. Therefore, the evolutionary analyses of secondary woodiness should include both the ontogenetic potential of herbaceous species and climatic niche shifts that promote this reversal.

Climate niche space explored by woody lineages is consistently smaller than that explored by sister lineages composed mainly of herbaceous taxa. Moreover, some data suggest that reversals to woodiness are accompanied by climatic niche shifts; such shifts may follow long-distance dispersals. This hypothesis explains the relatively high frequency of secondarily woody species on oceanic islands. It has not yet been formally tested.

In primarily woody plants, primary and secondary xylem differ with respect to several characters and those ontogenetic differences more or less reflect the evolutionary sequence of appearance: primitive characters are often present in early forming xylem while advanced (derived) characters usually occur in secondary xylem. In contrast, in secondarily woody angiosperms secondary xylem often has some juvenile characters suggesting that the reversal to woodiness is incomplete. This phenomenon is described as protracted juvenilism or paedomorphosis (heterochrony) in secondary xylem. However, in Apiaceae subfamily Apioideae most secondarily woody species do not exhibit protracted juvenilism in their wood. This suggests that throughout most of their evolutionary history herbaceous umbellifers have retained the ability of full reversal to woodiness. Effectively, ontogenetic constraints do not seem to limit the evolution of secondary woodiness in Apioideae. This hypothesis will be verified in this study (see more).