Towards the systematics and evolution of the conifer family Podocarpaceae; New insights into the key aspects

Abstract

Conifers are economically and ecologically important and form extensive forests in both the Southern and Northern Hemispheres. Southern Hemisphere is dominated by the conifer family Podocarpaceae which demonstrates remarkable morphological diversity in both leaves and seed cones. The results of this thesis focus on the systematics and evolutionary aspects of Podocarpaceae using molecular and seed cone morpho-anatomical aspects ranging from family to species level. In this thesis gaps in the literature and in our understanding of the Podocarpaceae have been investigated and presented in three main parts and nine chapters. The thesis begins (chapter 2) with an overview of the diversity, systematics, ecophysiological and ecological aspects of the Podocarpaceae. The family today consist of 20 genera and about 219 taxa (201 species, 2 subspecies, 14 varieties and 2 hybrids) placed in three clades and a paraphyletic group of four distinct genera. The data available on IUCN conservation status shows that 8 species are Critically Endangered (CR), and 27 species are Endangered (EN). Australasia (New Caledonia, Tasmania, New Zealand, and Malesia) is the hotspot of current Podocarpaceae diversity. Podocarp macrofossils are also predominantly recorded from Australia and New Zealand. The second part of the study investigates and describes the detailed seed cone morpho-anatomy of Podocarpaceae (chapters 3, 4 and 5). Most podocarp genera produce morpho-anatomically diverse seed cones and demonstrates diversity in structural traits. Based on the seed cone morpho-anatomical traits, chapter 6 explores the complexity of convergent evolution of fleshy cones in the Podocarpaceae. The structural morpho-anatomy demonstrated that podocarps have evolved fleshiness through several functional structures (free epimatium, fused epimatium, fleshy bracts, arils, and the receptaculum) that are involved in the formation of their fleshy cones. Each of these structures has distinct evolutionary patterns, involving multiple origins and/or losses of fleshiness, with multiple independent evolutionary changes either from non-fleshy to fleshy or fleshy to non-fleshy structures. The reconstruction of the ancestral Podocarpaceae seed cone is complicated by extinction, convergent evolution, and the relatively few fossil seed cone records. In chapter 7, the detailed reproductive morpho-anatomy of the Australian endemic conifer (Podocarpus lawrencei - Mountain Plum Pine) is described and compared with the species of the Australis clade. The reproductive morpho-anatomy shows the close similarity of Podocarpus lawrencei with P. nivalis in particular and no significant difference with other Australis clade species. In the last part of the thesis (chapter 8), a newly designed conifer bait set (appendix 1) was used to understand the biogeography of the Australis clade and genetic structure between the mainland and Tasmanian Podocarpus lawrencei populations. Both chloroplast and nuclear data supported the genetic divergence between the mainland southeastern Australian and Tasmanian populations. The historical biogeography and evolutionary history of the Australis clade suggest multiple extinctions, dispersal and adaptive evolution. The overall work in this thesis demonstrates that comprehensive morpho-anatomical and molecular studies are useful to address challenging trait evolutionary and phylogenetic questions. Further exploration of macrofossils, biogeographic, trait based evolutionary and phylogenomic studies in future will provide a better understanding of Podocarpaceae and conifer evolution.