Gene structure dynamics and divergence of the polygalacturonase gene family of plants and fungus.

Whole copies of the polygalacturonase (PG) genes from rice (Oryza sativa subsp. japonica) and a filamentous fungus (Aspergillus oryzae) were isolated. The orthologs of the rice PGs were also retrieved from other plant species. The 106 plant PGs analyzed were divided into 5 clades, A, B, C, D, and E. The fungus PGs were classified into 3 clades, of which one formed a loose cluster with clade E of the plant PGs. Four domain motifs (I, II, III, IV) were identified in all PGs. Motifs II and III were split by introns such as G/DDC and CGPGHGIS/IGSLG, respectively. In plant PGs there were 446 introns in total and 3.98 introns per gene. Intron phase distribution was 65.5% for phase 0, 19.7% for phase 1, and 14.8% for phase 2 in plant PGs. In the PGs of A. oryzae there were 37 introns of phase 0 (59.5%), phase 1 (24.3%), and phase 2 (16.2%), with 2.47 introns per gene. The 5 clades of plant PGs were divided into 3 basic gene structure lineages. Intron positions and phases were conserved among the PGs in the first 2 lineages. The third lineage consisted of PGs of clade E, which also carried highly conserved introns at different positions from other PGs. Intron positions were not as highly conserved in fungus PGs as in plant PGs. The introns in the current PGs have been present since before the divergence of monocots from dicots. The results obtained show that differential losses of introns created gene diversity, which was followed by segmental and tandem duplication in plant PGs.