U12-type spliceosomal introns of Insecta

Abstract

Most of eukaryotic genes are interrupted by introns that need to be removed from pre-mRNAs before they can perform their function. This is done by complex machinery called spliceosome. Many eukaryotes possess two separate spliceosomal systems that process separate sets of introns. The major (U2) spliceosome removes majority of introns, while minute fraction of intron repertoire is processed by the minor (U12) spliceosome. These two populations of introns are called U2-type and U12-type, respectively. The latter fall into two subtypes based on the terminal dinucleotides. The minor spliceosomal system has been lost independently in some lineages, while in some others few U12-type introns persist. We investigated twenty insect genomes in order to better understand the evolutionary dynamics of U12-type introns. Our work confirms dramatic drop of U12-type introns in Diptera, leaving these genomes just with a handful cases. This is mostly the result of intron deletion, but in a number of dipteral cases, minor type introns were switched to a major type, as well. Insect genes that harbor U12-type introns belong to several functional categories among which proteins binding ions and nucleic acids are enriched and these few categories are also overrepresented among these genes that preserved minor type introns in Diptera.

Keywords: U12-type introns; insect evolution.; minor spliceosome.

Figure 1

Number of U12-type introns in twenty insect genomes. Blue bars indicate Diptera order including Drosophilidae and Culicidae families, light and dark shade, respectively; green bars represent genomes of Hymenoptera order and the red bars represent neither Diptera nor Hymenoptera orders.

Figure 2

Evolutionary fate of seventy ancestral insect U12-type introns. Red numbers indicate number of deletions in a given lineage, while numbers in blue represent U12 to U2 intron type conversions.

Figure 3

Functional categories over-represented in genes containing the U12-type introns. A. Seventy insect genes. B. Twenty-five dipteral genes.