Heterogeneity of intron presence/absence in Luticola sp. (Bacillariophyta) contributes to the understanding of intron loss.

Although hypotheses have been proposed and developed to interpret the origins and functions of introns, substantial controversies remain about the mechanism of intron evolution. The availability of introns in the intermediate state is quite helpful for resolving this debate. In this study, a new strain of diatom (denominated as DB21-1) was isolated and identified as Luticola sp., in which multiple types of 18S rDNAs (obtained from genomic DNA; lengths ranged from 2,056 bp to 2,988 bp) were obtained. Based on the alignment between 18S rDNAs and 18S rRNA (obtained from cDNA; 1,783 bp), 7 intron insertion sites (IISs) located in the 18S rDNA were identified, each of which displaying the polymorphism of intron presence/absence. Specific primers around each IIS were designed to amplify the introns and the results indicated that introns in the same IIS varied in lengths but terminal sequences were conserved. Our study showed that the process of intron loss happens via a series of successive steps, and each step could derive corresponding introns under intermediate states. Moreover, these results indicate that the mechanism of genomic deletion that occurs at DNA level can also lead to exact intron loss. This article is protected by copyright. All rights reserved.