Question

Ancient whole genome duplications have occurred multiple times in the evolution of angiosperms. How might whole genome duplication have impacted the evolution of angiosperms? You can either speak generally or choose one of the events we talked about in class to answer the question.

Answer 1

Whole Genome Duplication (WGD) events have likely played a vital role in the evolutionary history of the angiosperms, both in terms of species richness and patterns of diversification. Many of the WGD events that have led to increased diversification rates appear to have occurred during a period of environmental instability, with one clear example being shortly after the Cretaceous-Paleogene boundary. Previous estimates of WGDs in angiosperms were in the range of at least 50 independent ancient events, but recent estimates push that number closer to 250 WGDs. Recent advancements in both genomic and transcriptomic sequencing have generated large data sets that have provided the necessary information to identify WGD events by comparing pairwise distances between paralogous genes in Ks plots. With newly identified WGD events, the goal is to determine whether these events were likely catalysts leading to diversification of the angiosperms. Angiosperms with multiple WGD events in their history have experienced higher speciation rates (and in the case of the rosids, higher net diversification rates), and (2) WGD events are associated with increases in diversification rates across the angiosperms.

Heterogenous speciation and rate shifts across the angiopsperms

The rates of speciation and extinction differed among subsets of the angiosperm tree. The overall speciation rate of the angiosperms is between 0.77 and 0.82, with extinction rates hovering around 0.65. Values for the monocot subset are similar, but the speciation rate of the asterids is much higher, 0.95–1.05, as well as being bimodal, and that of the rosids is much lower than the angiosperm-wide rate. The different rates of speciation and net diversification among clades of angiosperms are not
surprising, with some clades falling above and others below the net diversification rate of the angiosperms as a whole. Indeed, the evolutionary history of the angiosperms does not show a homogeneous trend, with slightly less than half of all species arising from lineages with low to moderate diversification rates and the rest from lineages with moderate to high diversification rates. These differences are most evident with herbaceous clades that show higher and more variable rates of molecular evolution. However, the interaction between traits, such as habit and diversification or WGD and diversification, is likely more complicated than current models can appear to correspond well with other time estimatesWith this in mind, 94 of 334 shifts (28.1%) occurred less than 5 Ma, with 149 events (44.6%) having occurred less than 10 Ma. Previous studies have shown that some parts of the angiosperm tree, such as Magnoliidae,
show an imbalanced distribution of species and shifts in net diversification across the clade

Polyploidy and diversification rates

Recent studies have attempted to assess the role of polyploidy in diversification. Synthesis among existing studies is difficult, in part due to varying scales of analysis. For example, in studies of a single genus or a single family, the species-level sampling may be complete or nearly complete, providing good resolution of shifts in diversification rates. However, results from these studies may not be widely applicable to all angiosperms, as diversification rates vary among clades, as noted above for monocots versus rosids versus asterids, and as reported previously. This type of focused approach was undertaken , when investigating diversification lag times in Nicotiana (Solanaceae) following allopolyploid events. Analyzing 55 of 75 species (73.3% of species in the genus) and six allopolyploid events, they found a 4–6-Myr lag time between WGD and changes in diversification rates. Considering that the ages of the WGD events in Nicotiana are much younger than the paleopolyploidy events that have shaped the radiation of the angiosperms as a whole and given variation in diversification patterns across angiosperms, these results, although informative regarding local examples of the lag-time model, may not be extensible to the angiosperms as a whole number of species in the clade) and 13 WGD events, found that even though some diversification rate shifts occurred following a WGD event, it did not appear that diversification was tightly linked
to WGD. Again, clade-specific patterns seem to vary, making synthesis difficult but perhaps suggesting that responses to WGD vary and that such events do not necessarily trigger increased rates of diversification.
Focusing on the angiosperms as a whole allows for discovery of widespread patterns, but comprehensive taxonomic sampling is, at this point, impossible: neither a time-calibrated tree nor sufficient WGD data exist for all angiosperms. Recent advancements have allowed further testing of WGD and diversification rates by dating the timing of the WGD events. Dating the two WGD events shared by all seed plants, as well as one shared
by all flowering plants, corroborate the lag-time effect of WGD and plant evolution, with a lag-time of approximately 50 Myr between duplication event and shifts in diversification. With more accurate dating of the 106 WGD events placed in this study, we may begin to investigate the proposed lag time hypothesis in a temporal framework more thoroughly, instead of via a node-specific manner as computed here. More precise dating of events may also help uncouple WGD, climatic/environmental conditions, and key innovations to determine which process or processes may be most important in determining the success of any given angiosperm lineage. This multifaceted approach would provide a more robust estimate of the amount of time that the lag-time hypothesis may require, perhaps to accommodate the many processes of genome downsizing and diploidization, prior to increased diversification rates in the evolutionary history of the angiosperms.