Natural transposable element insertions contribute to host fitness in model yeasts.

Transposable elements (TEs) are ubiquitous in the eukaryote genomes, but their evolutionary and functional significance remains largely obscure and contentious. Here, we explore the evolution and functional impact of TEs in two model unicellular eukaryotes, the fission yeast Schizosaccharomyces pombe and the budding yeast Saccharomyces cerevisiae, which diverged around 330-420 million years ago. We analyze the distribution of LTR retrotransposons (LTR-RTs, the only TE order identified in both species) and their solo-LTR derivatives in 35 strains of S. pombe and 128 strains of S. cerevisiae. We find that natural LTR-RT and solo-LTR insertions exhibit high presence-absence polymorphism among individuals in both species. Population genetics analyses show that solo-LTR insertions experienced functional constraints similar to synonymous sites of host genes in both species, indicating a majority of solo-LTR insertions might have evolved in a neutral manner. When knocking out 9 representative solo-LTR insertions separately in the S. pombe strain 972h- and 12 representative solo-LTR insertions separately in the S. cerevisiae strain S288C, we find that one solo-LTR insertion in S. pombe has significant effect on the fitness and transcriptomes of its host. Together, our findings indicate that a fraction of natural TE insertions likely shape their host transcriptomes and thereby contribute to their host fitness, with implications for understanding the functional significance of TEs in eukaryotes.

Phylogenetic analysis of achaete-scute complex genes in metazoans.

Achaete-scute complex (ASC) genes play essential roles in regulating neurogenesis of metazoans. Various metazoan species have greatly different numbers of genes in ASCa, ASCb and ASCc families. To explore evolutionary mechanisms of metazoan ASC genes, Blast (basic local alignment search tool) searches and phylogenetic analyses were conducted to identify ASC genes in metazoan species and to infer phylogenetic relationship between various ASC genes. As a result, 2784 ASC genes were identified in 804 metazoan species. The phylogenetic tree constructed using 1237 unique bHLH motifs shows that metazoan ASCa, ASCb and ASCc families contain six (a1-a6), five (b1-b5) and three (c1-c3) bHLH genes, respectively. Further phylogenetic analyses suggest that ASC genes in metazoans are derived from a primitive c gene, those in insects are derived from c2 gene, and those in chordates are derived from a2 and a3 genes. Data of gene linkage demonstrate that insect a6 is derived from a4 but not from a5, and chordate a2 is ancestral to b5 only, whilst a3 is ancestral to both b3 and b5. It is concluded that current ASC gene families in metazoans were established through a series of sub- and/or neo-functionalization to duplicated ancestral ASC gene(s). These results provide good references for exploring evolutionary mechanisms of other bHLH genes in metazoans. Besides, gene subtyping is considered as an efficient method for evolutionary studies on closely related homologous genes.

Simulation of Chordate Intron Evolution Using Randomly Generated and Mutated Base Sequences.

Introns are well known for their high variation not only in length but also in base sequence. The evolution of intron sequences has aroused broad interest in the past decades. However, very little is known about the evolutionary pattern of introns due to the lack of efficient analytical method. In this study, we designed 2 evolutionary models, that is, mutation-and-deletion (MD) and mutation-and-insertion (MI), to simulate intron evolution using randomly generated and mutated bases by referencing to the phylogenetic tree constructed using 14 chordate introns from TF4 (transcription factor-like protein 4) gene. A comparison of attributes between model-generated sequences and chordate introns showed that the MD model with proper parameter settings could generate sequences that have attributes matchable to chordate introns, whereas the MI model with any parameter settings failed in doing so. These data suggest that the surveyed chordate introns have evolved from a long ancestral sequence through gradual reduction in length. The established methodology provides an effective measure to study the evolutionary pattern of intron sequences from organisms of various taxonomic groups. (C++ scripts of MD and MI models are available upon request.).