On the origins of species: does evolution repeat itself in polyploid populations of independent origin?

Multiple origins of the same polyploid species pose the question: Does evolution repeat itself in these independently formed lineages? Tragopogon is a unique evolutionary model for the study of recent and recurrent allopolyploidy. The allotetraploids T. mirus (T. dubius x T. porrifolius) and T. miscellus (T. dubius x T. pratensis) formed repeatedly following the introduction of three diploids to the United States. Concerted evolution has consistently occurred in the same direction (resulting in loss of T. dubius rDNA copies). Both allotetraploids exhibit homeolog loss, with the same genes consistently showing loss, and homeologs of T. dubius preferentially lost in both allotetraploids. We have also documented repeated patterns of tissue-specific silencing in multiple populations of T. miscellus. Hence, some aspects of genome evolution may be "hardwired," although the general pattern of loss is stochastic within any given population. On the basis of the study of F(1) hybrids and synthetics, duplicate gene loss and silencing do not occur immediately following hybridization or polyploidization, but gradually and haphazardly. Genomic approaches permit analysis of hundreds of loci to assess the frequency of homeolog loss and changes in gene expression. This methodology is particularly promising for groups such as Tragopogon for which limited genetic and genomic resources are available.

Differential responses of germination and seedling establishment in populations of Tragopogon pratensis (Asteraceae).

The establishment phase is an important bottleneck in the life cycle of plants. It consists of two steps that are rarely separated, i.e., the germination of seeds and the establishment of seedlings. Here we report the results of two experiments in which we independently investigated germination and seedling establishment in the greenhouse, under different grass vegetation treatments representing different regeneration niches. Seeds of Tragopogon pratensis from six populations and two habitat types were studied, three from roadside verges and three from hayfields. Germination percentages and germination speed were higher for seeds from roadside verges than for seeds from hayfields, but were little affected by treatment. In contrast, seedling growth was much lower in the tall grass vegetation, than in the short grass and especially the bare soil treatment. Seedling sizes were generally similar for different populations and habitat types. Our results thus show that the two early steps in the establishment phase of plants may respond very differently to the micro-environment, and may have a different selection history. Insight into the ecology and evolution of life histories may require that germination and establishment are considered separately.