In the Light of Evolution Volume III: Two Centuries of Darwin (2009) / Chapter Skim
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2 Adaptive Radiations:From Field to Genomic Studies--Scott A. Hodges and Nathan J. Derieg
2 Adaptive Radiations:From Field to Genomic Studies--Scott A. Hodges and Nathan J. Derieg
Pages 27-46
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From page 27... ...
Here, we review the advantages of adaptive radiations, especially recent ones, for detecting evolutionary trends and the genetic dissection of adaptive traits. We focus on Aquilegia as a primary example of these advantages and highlight progress in understanding the genetic basis of flower color.
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From page 28... ...
Key to Darwin's suspicion of common ancestry was the close proximity of environmentally similar islands comprising the Galápagos archipelago. in fact, many classic examples of adaptive radiations involve islands or lakes; notable examples include Darwin's finches of the Galápagos, honeycreeper birds and silversword plants of hawaii, and cichlid fish of lakes Malawi and victoria in Africa.
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From page 29... ...
] , whereas studies of adaptive radiations have focused attention on the role of divergent natural selection to alternate environments as a primary cause of reproductive isolation (schluter, 1996, 2000, 2001; rundle et al., 2000; via, 2001)
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From page 30... ...
second, recent adaptive radiations often consist of taxa that can be hybridized, thus making possible the genetic dissection of traits. Third, adaptive radiations may entail a diversity of adaptive traits or the repeated evolution of the same traits in different lineages providing multiple comparisons within and between traits in a single system.
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From page 31... ...
We then highlight how genomic studies are aiding in our understanding of the genetic basis of adaptation and speciation, emphasizing flower color as a primary model. We end with a look toward how the genetic dissection of more complex traits will be possible in the near future.
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From page 32... ...
. These shifts in color appear to be largely adaptive because all 5 inferred shifts to hawkmoth pollination are coincident with loss of anthocyanin production (Whittall et al., FiGUre 2.1 Photographs of Aquilegia flowers.
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From page 33... ...
. Arrows indicate down-regulation of genes late in the core anthocyanin pathway in flowers of that species compared with the regulation in the anthocyanin-producing species A
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From page 34... ...
Thus, color shifts in Aquilegia, where anthocyanin production is maintained, do not exhibit an evolutionary trend. FLOWER COLOR AND THE GENETICS OF ADAPTATION The significant trends in pollinator evolution in Aquilegia, together with field experimentation on the functional relevance of specific traits, make variation in spur length, flower orientation, and flower color obvious targets for understanding the genetic basis of adaptive traits and traits affecting reproductive isolation.
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From page 35... ...
has described general trends in flower color evolution: shifts are generally blue to red or from producing anthocyanin to not, although exceptions do occur. Considering the biochemical pathway for anthocyanins, rausher pointed out that these trends likely arise because mutations causing loss of function are more likely than those causing
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From page 36... ...
in Petunia axillaris, white flowers can be made pink by the introduction, either through introgression or transgenetics, of a functional copy of AN2, the r2r3-myb transcription factor that controls expression of genes late in the core ABP (Fig.
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From page 37... ...
We know of no study that also has considered how changes in flux from side branches of the core ABP may have affected anthocyanin production in natural systems. AQUILEGIA AS A MODEL FOR STUDYING FLORAL COLOR EVOLUTION Aquilegia has been used for nearly 50 years to study the evolution and genetics of flower color.
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From page 38... ...
. These studies suggest that losses of floral anthocyanins are likely caused by changes in expression patterns of the core enzymatic genes of the ABP or changes causing substrate flux away from anthocyanin production.
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From page 39... ...
From these initial searches, we used the strongest Aquilegia hits in tblastx searches of The Arabidopsis information resource and national Center for Biotechnology information, and those that resulted in best hits to the original characterized gene were retained as potential flavonoid pathway genes. several genes in the flavonoid pathway belong to large multigene families, making the criterion of reciprocal best hit for identifying homologous genes suspect.
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From page 40... ...
found only 3- and 5-glucosides and 3,5-diglucoside classes of anthocyanins in Aquilegia. Using the Aquilegia gene index we identified candidates for nearly all major genes in the flavonoid pathway (Table 2.1 and Fig.
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From page 41... ...
Functional characterizations, other than for Arabidopsis genes, can be found in Mathews et al.
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From page 42... ...
our analysis revealed 27 candidates for enzymatic genes in the flavonoid pathway and 7 candidates for transcription factors regulating the core anthocyanin pathway. interestingly, 4 genes of the core ABP (Chi, F3h, DFr, and Ans)
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From page 43... ...
. of course, there may be even more genes involved in the flavonoid pathway as the Aquilegia gene index likely does not contain all paralogs of the ABP genes and additional side-branch enzymes are possible (yonekura-sakakibara et al., 2008)
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From page 44... ...
Genomic resources will also greatly aid in our ability to dissect traits about which we currently understand little in terms of underlying biochemistry and genetics. Although traits such as petal spur length and flower orientation have strong effects on pollinator visitation and resulting pollen transfer (hodges et al., 2004)
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From page 45... ...
For the reasons outlined above, adaptive radiations maximize our ability to detect patterns and test both long-standing and emerging hypotheses about the nature of adaptive evolutionary change. ACKNOWLEDGMENTS We thank John Avise and Francisco Ayala for the opportunity to discuss our work at the sackler Colloquium and John Avise, Dolph schluter, and an anonymous referee for providing comments that greatly improved the manuscript.
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