Genome-Wide Identification and Evolutionary Analysis of the Fruit-Weight 2.2-Like Gene Family in Polyploid Oilseed Rape (Brassica napus L.)

The fruit-weight 2.2 (fw2.2) locus, which was first described in tomato, is known for controlling up to 30% of fruit mass. The functions of its homologs, the FW2.2-like (FWL) genes, have also been documented in other diploid plants such as maize and rice. However, the evolution and contribution of the FWL gene family to seed weight in polyploid crops remain to be explored. In this study, we deployed an integrated approach to characterize the FWL gene family in the allotetraploid crop, Brassica napus. A total of 18 FWL genes were identified and designated BnFWL1-18. These were classified into three groups based on their phylogenetic relationships, which were supported by multiple sequence alignment, chromosome location, collinearity, transmembrane prediction, conserved motifs, selection pressure, protein three-dimensional (3D) structure, and the composition and position of cis-regulatory elements. Strikingly, three conserved 3D models were identified in all 18 BnFWL proteins, pertaining to the soluble N-ethylmaleimide-sensitive factor attachment protein receptor complex. Intriguingly, we found that the BnFWL12 protein was structurally similar to phosphoenolpyruvate carboxylase, which is required for photosynthesis. More importantly, we identified an "SDGE" phosphorylation motif in BnFWL12 in a search for putative casein kinase II (CK2) phosphorylation sites. Moreover, the temporospatial expression profiles in different tissues revealed that the discrete expression patterns are likely associated with the functional differentiation of BnFWL genes. Taken together, our data shed light on the evolutionary divergence of the FWL gene family in a polyploid crop, providing foundations for further functional studies.