Intron Sliding and Length Variability of Genes Enriched of Phase 1 Long Introns

Due to high mutagenesis of intron sequences, intron evolution is usually considered in terms of evolution of exon-intron structures (EIS). The shifting of intron over short distances (rare evolutionary event called intron sliding) could lead to the change of intron phase, i.e. the intron position relative to the open reading frame. Here we analyze the EIS from four datasets of eukaryotic orthologues in order to find out the preferable choice of intron phase during sliding and to study the correlation between orthologous intron lengths. To identify the orthologous introns we have constructed the alignments of EIS of orthologous genes. Several sliding events with intron phase change were revealed from the analysis; however, our initial hypothesis that in the process of sliding introns prefer to change its phase to 0 more frequently, was not been confirmed. Nevertheless, it is necessary to expand the analysis on a larger dataset for making a proper conclusions. Despite high variability of intron length, some taxonomic groups share the similar length values. Moreover, some length conservation could be observed if instead of intron length L we consider a normalized length N = (L-A)/A, where A is an average length within an orthologous intron group. E.g. for ptprd genes of birds (28 species) the normalized value is in the interval (-0.15, 0.15) for 85.2 % of introns what is significantly higher than the values for random lengths set in accordance with the intron lengths distribution. That length “conservation” leads us to the question what intron length was in the ancient introns.

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.