Growth form evolution in piperales and its relevance for understanding angiosperm diversification : an integrative approach combining plant architecture, anatomy, and biomechanics

A striking feature of early angiosperm lineages is the variety of life forms and growth forms, which ranges from herbs, aquatic herbs, climbers, and epiphytes to woody shrubs and trees. This morphological and anatomical diversity is arguably one of the factors explaining how angiosperms dominate many ecosystems worldwide. However, just how such a wide spectrum of growth forms has evolved in angiosperms remains unclear. In this review, we investigate patterns of growth form diversification in Piperales, an early-diverging lineage (with stem age estimated at 201-128 Myr ago) and the most morphologically diverse clade among magnoliids. We outline patterns of growth form diversity and architecture as well as the biomechanical significance of developmental characters, such the organization, loss, and gain of woodiness. Asaroideae and Saururaceae are terrestrial as well as semiaquatic to aquatic herbaceous perennials bearing rhizomes. The Aristolochioideae and Piperaceae show higher levels of growth form diversity and biomechanical organization, with complex patterns of increasing or decreasing woodiness and architectural organization. The climbing habit has probably evolved independently in the Aristolochiaceae and Piperaceae, while mechanically unstable shrubs and, less frequently, treelets have evolved several times within these two most species-rich clades. A key developmental character underlying diversity in most Piperales-with the exception of the herbaceous Saruma (Asaroideae)-is the conserved development of the wood cylinder, in which fusiform initials are limited to fascicular carnbial initials. The resulting large fraction of raylike tissue in the stem-a highly characteristic feature of woody species in the Piperales-potentially introduced mechanical constraints on the diversification of self-supporting architectures. This was possibly circumvented by the architectural development of repeated, large-diameter meristems in some shrublike habits via sympodial growth. Patterns of growth form evolution within Piperales potentially mirror some of the overall trends observed among early-diverging angiosperms as a whole as well as angiosperms in general. These include profound changes in life form and growth form linked to large-scale transitions in woodiness, diversity of mechanical organization, and shifts in architectural development

The genetics of evolutionary radiations

With the realization that much of the biological diversity on Earth has been generated by discrete evolutionary radiations, there has been a rapid increase in research into the biotic (key innovations) and abiotic (key environments) circumstances in which such radiations took place. Here we focus on the potential importance of population genetic structure and trait genetic architecture in explaining radiations. We propose a verbal model describing the stages of an evolutionary radiation: first invading a suitable adaptive zone and expanding both spatially and ecologically through this zone; secondly, diverging genetically into numerous distinct populations; and, finally, speciating. There are numerous examples of the first stage; the difficulty, however, is explaining how genetic diversification can take place from the establishment of a, presumably, genetically depauperate population in a new adaptive zone. We explore the potential roles of epigenetics and transposable elements (TEs), of neutral process such as genetic drift in combination with trait genetic architecture, of gene flow limitation through isolation by distance (IBD), isolation by ecology and isolation by colonization, the possible role of intra-specific competition, and that of admixture and hybridization in increasing the genetic diversity of the founding populations. We show that many of the predictions of this model are corroborated. Most radiations occur in complex adaptive zones, which facilitate the establishment of many small populations exposed to genetic drift and divergent selection. We also show that many radiations (especially those resulting from long-distance dispersal) were established by polyploid lineages, and that many radiating lineages have small genome sizes. However, there are several other predictions which are not (yet) possible to test: that epigenetics has played a role in radiations, that radiations occur more frequently in clades with small gene flow distances, or that the ancestors of radiations had large fundamental niches. At least some of these may be testable in the future as more genome and epigenome data become available. The implication of this model is that many radiations may be hard polytomies because the genetic divergence leading to speciation happens within a very short time, and that the divergence history may be further obscured by hybridization. Furthermore, it suggests that only lineages with the appropriate genetic architecture will be able to radiate, and that such a radiation will happen in a meta-population environment. Understanding the genetic architecture of a lineage may be an essential part of accounting for why some lineages radiate, and some do not