From Pangea till date: gymnosperm evolution, biogeography and phylogenetics

 

Maximum likelihood tree os 739 gymnosperm taxaBurgleigh J. G. et al. Exploring diversification and genome size evolution in extant gymnosperms through phylogenetic synthesis. Journal of Botany Vol. 2012: Article ID 292857, 6 pages. doi:10.1155/2012/292857. This study first synthesizes the available phylogenetically informative sequences to build a phylogenetic hypothesis of gymnosperms that reflects the recent advances in sequencing and computational phylogenetics. The authors use the resulting tree to examine large-scale patterns of diversification of the extant gymnosperm lineages and also to examine rates of genome size evolution. Their analyses of gymnosperm diversification and genome size evolution demonstrate that the extant gymnosperms are a vibrant, growing clade, and not simply the sole survivors of ancient diversity. Furthermore, Pinus is unique among gymnosperms. That is, the highly elevated rates of change in genome size appear to be limited to Pinus. Although Pinus is a species-rich genus, they could not find any links between increased rates of diversification and shifts in rates of genome size evolution.

Dated phylogeny for 489 extant conifer species with cycad as an outgroupLeslie A. B. et al. Hemisphere-scale differences in conifer evolutionary dynamics. P. Natl. Acad. Sci. USA 2012. 109(40): 16217-16221. To explore the impact of biogeographic differences on the evolutionary history of conifers, they incorporated genetic information with a review of fossil evidence to construct an age calibrated phylogeny sampling about 80% of living conifer species. Their results show that most extant conifer species diverged recently during the Neogene within clades that generally were established during the later Mesozoic, but lineages that diversified mainly in the Southern Hemisphere show a significantly older distribution of divergence ages than their counterparts in the Northern Hemisphere. Their analyses demonstrate significant differences in the diversification of conifer clades inhabiting the Northern and Southern Hemispheres over the last 65 million years. General differences in climatic and landscape history resulting from the dispositions of landmasses in the hemispheres appear to have left a distinct imprint on conifer evolutionary history.

Inferred dispersal routes of gymnospermous genera with intercontinental distribution_Wangandran_molphylandevol_2014Wang X.Q. and Ran J.H. Evolution and biogeography of gymnosperms. Mol. Phylogenet. Evol. 2014. 75: 24-40. The authors have reviewed recent advances in understanding of gymnosperm evolution and biogeography, including phylogenetic relationships at different taxonomic levels, patterns of species diversification, roles of vicariance and dispersal in development of intercontinental disjunctions, modes of molecular evolution in different genomes and lineages, and mechanisms underlying the formation of large nuclear genomes.

Chaw S. M. et al. Molecular phylogeny of extant gymnosperms and seed plant evolution: analysis of nuclear 18S rRNA sequences. Mol. Biol. Evol. 1997. 14(1): 56-68. Their phylogenetic analyses indicate that the living seed plants are monphyletic clade. Modern seed plants share a common ancestor and the seed evolved only once. Moreover, the extant gymnosperms are ancestor of the angiosperms and that the diversification of modern gymnosperms probably did not occur until the split between the gymnosperms and angiosperms had taken place.

Genome evolution in angio and gymnosperms_leitchandleitch_NewPhytologist_2012Leitch A. R. and Leitch I. J. Ecological and genetic factors linked to contrasting genome dynamics in seed plants. New Phytol. 2012. 194(3): 629-646. Authors have reviewed the genomic and ecological factors that have shaped the seed plant genomes. They have compared and contrasted the genome structure and evolution of angiosperms and gymnosperms. They observe that although the angiosperms have been undergoing relatively fast genome evolution compared with gymnosperms, the gymnosperms have evolved genetic novelties not encountered in either the angiosperms or other vascular plants (i.e. monilophytes and lycophytes). Therefore, they suggest that genomes in these two groups have undergone distinct evolutionary trajectories, and the fundamental differences in life strategy and genome dynamism could have resulted in angiosperms becoming the dominant land plant group.

Other interesting papers:

Ahuja M. R. and Neale D. Evolution of genome size in conifers. Silvae Genetica 2005. 54 (3): 126-137. The authors review intra- and interspecific genome size variation, and mechanisms of genome size expansion, contraction and evolution.

Morse A. M. et al. Evolution of Genome Size and Complexity in Pinus.PLoS One 2009 4(2): e4332. doi:10.1371/journal.pone.0004332. Certain classes of repeat elements show distinct chromosomal distributions in angiosperms and gymnosperms and epigenetic markings associated with heterochromatin differ in angiosperms and gymnosperms. Determining whether gymnosperms share a similar distribution of elements, or exhibit a distinct genomic architecture, is a key to understanding how evolution has shaped these two major lineages of seed plant. Most of the enormous genome complexity of pines can be explained by divergence of retrotransposons, however the elements responsible for genome size variation are yet to be identified. Genomic resources for Pinus including those reported in this paper should assist in further defining whether and how the roles of retrotransposons differ in the evolution of angiosperm and gymnosperm genomes.

Pavy N. et al. A spruce gene map infers ancient plant genome reshuffling and subsequent slow evolution in the gymnosperm lineage leading to extant conifers. BMC Biology 2012. 10: 84. The authors found that much genomic evolution has occurred in the seed plant lineage before the split between gymnosperms and angiosperms, and that the pace of evolution of the genome macro-structure has been much slower in the gymnosperm lineage leading to extent conifers than that seen for the same period of time in flowering plants. This trend is largely congruent with the contrasted rates of diversification and morphological evolution observed between these two groups of seed plants.

 

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