Comparative evolution history of SINEs in Arabidopsis thaliana and Brassica oleracea: evidence for a high rate of SINE loss.

Brassica oleracea and Arabidopsis thaliana belong to the Brassicaceae(Cruciferae) family and diverged 16 to 19 million years ago. Although the genome size of B. oleracea (approximately 600 million base pairs) is more than four times that of A. thaliana (approximately 130 million base pairs), their gene content is believed to be very similar with more than 85% sequence identity in the coding region. Therefore, this important difference in genome size is likely to reflect a different rate of non-coding DNA accumulation. Transposable elements (TEs) constitute a major fraction of non-coding DNA in plant species. A different rate in TE accumulation between two closely related species can result in significant genome size variations in a short evolutionary period. Short interspersed elements (SINEs) are non-autonomous retroposons that have invaded the genome of most eukaryote species. Several SINE families are present in B. oleracea and A. thaliana and we found that two of them (called RathE1 and RathE2) are present in both species. In this study, the tempo of evolution of RathE1 and RathE2 SINE families in both species was compared. We observed that most B. oleracea RathE2 SINEs are "young" (close to the consensus sequence) and abundant while elements from this family are more degenerated and much less abundant in A. thaliana. However, the situation is different for the RathE1 SINE family for which the youngest elements are found in A. thaliana. Surprisingly, no SINE was found to occupy the same (orthologous) genomic locus in both species suggesting that either these SINE families were not amplified at a significant rate in the common ancestor of the two species or that older elements were lost and only the recent (lineage-specific) insertions remain. To test this latter hypothesis, loci containing a recently inserted SINE in the A. thaliana col-0 ecotype were selected and characterized in several other A. thaliana ecotypes. In addition to the expected SINE containing allele and the pre-integrative allele (i.e. the "empty" allele), we observed in the different ecotypes, alleles with truncated portions of the SINE (up to the complete loss of the element) and of the immediate genomic flanking sequences. The absence of SINEs in orthologous positions between B. oleracea and A. thaliana and the presence in recently diverged A. thaliana ecotypes of alleles containing severely truncated SINEs suggest a very high rate of SINE loss in these species.

Lenoir A ，Pélissier T ，Bousquet-Antonelli C ，Deragon JM ... -  《-》

The evolutionary origin and genomic organization of SINEs in Arabidopsis thaliana.

Lenoir A ，Lavie L ，Prieto JL ，Goubely C ，Coté JC ，Pélissier T ，Deragon JM ... -  《MOLECULAR BIOLOGY AND EVOLUTION》

Short interspersed elements (SINEs) in plants: origin, classification, and use as phylogenetic markers.

Deragon JM ，Zhang X 《SYSTEMATIC BIOLOGY》

Novel SINE families from salmons validate Parahucho (Salmonidae) as a distinct genus and give evidence that SINEs can incorporate LINE-related 3'-tails of other SINEs.

Short interspersed elements (SINEs) constitute a group of retroposons propagating in the genome via a mechanism of reverse transcription, in which they depend on the enzymatic machinery of long retroposons (LINEs). Over 70 SINE families have been described to date from the genomes of various eukaryotes. Here, we characterize two novel SINEs from salmons (Actinopterygii: Salmonoidei). The first family, termed SlmI, was shown to be widespread among all genera of the suborder. These SINEs have a tRNA(Leu)-related promoter region at their 5'-end, a unique central conserved domain with a subfamily-specific region, and an end with RSg-1-LINE-derived 3'-terminus preceding the A/T-rich tail. The same LINE-related segment is also shared by two other salmonid SINEs: HpaI and OS-SINE1. The structural peculiarities and overall sequence identity of the SlmI 3'-terminus suggest that it has been acquired from HpaI SINEs but not directly from the partner LINE. This region plays a crucial role in the process of retrotransposition of short interspersed elements, and the case of its SINE-to-SINE transmission is the first recorded to date. Possible scenarios and potential evolutionary implications of the observed interaction between short retroposons are discussed. Apart from the above, we found a copy of the SlmI SINE in the GenBank entry for the blood fluke, Schistosoma japonicum (Trematoda: Strigeiformes) -- a trematode causing one of the most important human helminth infections, with its genome known to host other groups of salmonoid retroposons. In the present article, we suggest our views with regard to possible ways in which such an intensive horizontal transfer of salmonoid retroposons to the schistosomal genome occurs. The second novel SINE family, termed SlmII, originates from one of the SlmI subfamilies, with which it shares the same tRNA-related region, central domain, and a part of RSg-1-derived segment, but has a different 3'-tail of unidentified origin. Its distribution among salmonids validates Parahucho (Japanese huchen) as a distinct monotypic genus.

Matveev V ，Nishihara H ，Okada N 《MOLECULAR BIOLOGY AND EVOLUTION》

The evolution of two partner LINE/SINE families and a full-length chromodomain-containing Ty3/Gypsy LTR element in the first reptilian genome of Anolis carolinensis.

Transposable elements have been characterized in a number of vertebrates, including whole genomes of mammals, birds, and fishes. The Anolis carolinensis draft assembly provides the first opportunity to study retroposons in a reptilian genome. Here, we identified and reconstructed a number of retroposons based on database searches: Five Sauria short interspersed element (SINE) subfamilies, 5S-Sauria SINE chimeras, Anolis Bov-B long interspersed element (LINE), Anolis SINE 2, Anolis LINE 2, Anolis LINE 1, Anolis CR 1, and a chromodomain-containing Ty3/Gypsy LTR element. We focused on two SINE families (Anolis Sauria SINE and Anolis SINE 2) and their partner LINE families (Anolis Bov-B LINE and Anolis LINE 2). We demonstrate that each SINE/LINE pair is distributed similarly and predict that the retrotransposition of evolutionarily younger Sauria SINE members is via younger Bov-B LINE members while a correlation also exists between their respective evolutionarily older SINE/LINE members. The evolutionarily youngest Sauria SINE sequences evolved as part of novel rolling-circle transposons. The evolutionary time frame when Bov-B LINEs and Sauria SINEs were less active in their retrotransposition is characterized by a high retrotransposition burst of Anolis SINE 2 and Anolis LINE 2 elements. We also characterized the first full-length chromoviral LTR element in amniotes (Amn-ichi). This newly identified chromovirus is widespread in the Anolis genome and has been very well preserved, indicating that it is still active. Transposable elements in the Anolis genome account for approximately 20% of the total DNA sequence, whereas the proportion is more than double that in many mammalian genomes in which such elements have important biological functions. Nevertheless, 20% transposable element coverage is sufficient to predict that Anolis retroposons and other mobile elements also may have biologically and evolutionarily relevant functions. The new SINEs and LINEs and other ubiquitous genomic elements characterized in the Anolis genome will prove very useful for studies in comparative genomics, phylogenetics, and functional genetics.

Piskurek O ，Nishihara H ，Okada N 《-》