Functional evolutionary studies of a cotton fiber initiation transcription factor

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The widely cultivated cotton species, Gossypium hirsutum (AD)1 is a major fiber producing crop, accounting for 90-95% of the world’s cotton. G. hirsutum is an allotetraploid species that originated from a polyploidy event 1-2 million years ago when the genomes of two diploid species [G. arboreum (A2), and G. raimondii (D5)] merged together. The parental A-genome cotton produces spinnable fiber, with lower quality and abundance than tetraploid cotton, while D-genome does not produce spinnable fiber. Of the 45 diploid species only A-diploid species produce spinnable fibers. It is uncertain, in the evolution of cotton species, when and how this spinnable fiber trait emerged in A-diploid species. Here, we employed the Arabidopsis (Arabidopsis thaliana) trichome model system to study the evolution of the spinnable cotton fiber production trait. The core trichome initiation complex genes (bHLH, WD40, and R2R3) were tested for the functional differences between A- and D-genomes, the parental species of the cultivated tetraploid species. The A- and D-species showed functional differences in two basic helix-loop-helix (bHLH) transcription factors (TFs), namely DEL65 and DEL61. The functional evolution of these factors was studied using the Arabidopsis model system. The study was extended further to understand the functional evolution of the bHLH transcription factor and to determine whether the bHLH transcription factor genes were evolved to be functional in A-genomes or lost the function in D-genomes. To achieve this, the bHLH transcription factors were amplified from different genome groups of the different clades of the cotton evolutionary tree (A, D, F, B, E, C, G, K, and Gossypoides kirkii, an out group). These were complemented in a trichome initiation defective Arabidopsis double mutant (gl3/egl3). The results from this study showed that the bHLH transcription factors were evolved to be functional in A-diploid. The G and K genomes also showed gain of function of the bHLH factor(s). This study will lead to a greater understanding of the gain of function of the bHLH transcription factor during the evolution of the cotton initiation trait of A-diploid genomes.

Trichome initiation complex, Cotton fiber initiation complex