GORDITA, a young paralog of Arabidopsis thaliana Bsister MADS-box gene ABS, has undergone neofunctionalization

Abstract

Bsister genes, a clade with close relationships to the class B floral homeotic genes, have been conserved for more than 300 million years. Bsister genes in Arabidopsis thaliana underwent gene duplication probably before the diversification of Brassicaceae leading to the paralogue genes ARABIDOPSIS BSISTER (ABS) and GORDITA (GOA). The phenotype of the abs mutant, however, is rather mild as it shows only reduced seed coloration and defects in endothelium development. This thesis focuses on the enigmatic GOA gene, which was suspected to act redundantly with ABS. Phylogeny reconstructions and selection analysis revealed that ABS was under purifying selection, while GOA has evolved under relaxed selection pressure after the duplication. The GOA knockdown and goa mutant phenotypes suggest a role for this gene in fruit longitudinal growth, while over-expression of GOA results in disorganized floral structure and the addition of carpel-like features to sepals. Protein analyses revealed that a part of the typical K- and the whole C-domain of MIKC-type MADS-box genes are replaced by a GOA-specific deviant (D)-domain, which is required for protein dimerization. The D-domain, unique among MIKC-type MADS-box genes, originated from the ABS sequence by several frame shifts, nucleotide exchanges, and changes of splice acceptor and donor sites. As consequence the GOA protein lost the putative alpha-helix structure (K3) in the K-domain. Interestingly, the GOA protein has evolved a new putative alpha-helix structure (KD) in the D-domain, which is essential for protein dimerization. Even though it is most expressed in flowers and fruits, GOA does not heterodimerize with other floral homeotic proteins but AGL16 and ABS. No shared protein interaction partners could be identified for ABS and GOA. A yeast two-hybrid screen using an all cDNA library from A. thaliana revealed putative groups of proteins as possible GOA interacting partners outside the MIKC-type protein family. These include transcription factors and chromatin remodelling enzymes that are involved in regulation of development, cell growth or cell proliferation. This work highlights that neofunctionalization following gene duplication can be achieved in a relatively short time by modification of protein domains and expression pattern. A completely opposite situation was found in Eschscholzia Californica where AGAMOUS (AG) duplicated and both paralogs retained. Expression analysis of B and C genes revealed a C gene dependent regulation of B genes.