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Molecular evolutionary analysis of TALE homeobox in Viridiplantae Wang, Ming Hsiu


The emergence of embryophytes from their charophyte-like ancestor is estimated to have occurred 476-432 MYA. During the adaptation to land, embryophytes evolved to have sporic meiosis; whereas charophyte algae undergo zygotic meiosis. The transition to land required the embryophytes to develop specialized tissues and a cuticle to survive drier terrestrial environments. This transition resulted in increasing elaboration of the body plan in the diploid phase, establishing the sporophyte. It is hypothesized that diversification of heterodimeric TALE homeobox genes in the ancestral charophyte algae may have acted as new types of master regulators to control diploid-specific developmental program, which initiated the development of novel sporophytic body plan. This study is focused on determining TALE homeobox genealogy by comparing genetic sequences and gene structure of TALE homeobox found in the transcriptomes of Picocystis salinarum (prasinophyte), Mougeotia sp. (charophyte). and Cosmocladium constrictum (charophyte). The interaction of TALE homeobox proteins from Picocystis salinarum was tested with a Y2H assay. Prior to this study, it was known that the diploid developmental program was regulated by KNOX and BELL classes of TALE homeobox genes in embryophytes and KNOX and GSP1 classes of TALE homeobox genes in Chlamydomonas reinhardtii (chlorophyte). Through phylogenetic analysis, I found that charophytes express KNOX, BELL and GSP1 classes, and P. salinarum expresses KNOX, GSP1, divergent TALE, and two red algal homologs of the TALE homeobox. Furthermore, comparison of intron location indicated that the BELL and GSP1 genes in the charophytes may be homologous. Intron comparisons and phylogenetic analysis of the KNOX genes indicate that KNOX II class from streptophyta and KNOX from chlorophyta share the greatest similarity, whereas KNOX I class can be hypothesized to have emerged by gene duplication in the early charophyte ancestor. The Y2H assay of TALE homeobox from Picocystis salinarum shows that GSP1 and KNOX can interact, whereas the possibility of an interaction with the red algal homolog is inconclusive.

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