The main difference between LTR retrotransposons and retroviruses is the presence of the envelope (env) gene in the latter, downstream of the pol gene. The env gene is involved in their infectious capacity. Here we report the presence of env-like sequences in the genome of Quercus suber (cork oak), one of the most economically important Portuguese species. These gene sequences were isolated through DNA amplification between RNaseH conserved motifs and 3' LTR, based on the structure of copia retrotransposons. Phylogenetic analysis revealed that almost all the clones isolated are clustered with Cyclops-2, a Ty3-gypsy element identified in Pisum sativum, except one clustered with gypsy and copia retroelements found in different species. This suggests the existence of a potential ancestral sequence of the env gene, prior to the separation of Ty3-gypsy and Ty1-copia retrotransposons. Additionally, the isolated env-like sequences showed 26?39% of homology with env-like sequences characterized in viruses. The origin of env-like sequences in retrotransposons from host plant taxa is discussed.
A repetitive sequence of 411 bp, named pSaO5411, was identified in the Secale africanum genome (Ra) by random amplified polymorphic DNA (RAPD) analysis of wheat and wheat?S. africanum amphiploids. GenBank BLAST search revealed that the sequence of pSaO5411 was highly homologous to a part of a Ty1-copia retrotransposon. Fluorescence in situ hybridization (FISH) analyses indicated that pSaO5411 was significantly hybridized to S. africanum chromosomes of a wheat?S. africanum amphiploid, and it was dispersed along the Secale chromosome arms except the terminal regions. Basing on the sequence of pSaO5411, a pair of sequence-characterized amplified region (SCAR) primers were designed, and the resultant SCAR marker was able to target both cultivated rye and the wild Secale species, which also enabled to identify effectively the S. africanum chromatin introduced into the wheat genome.
Fast and efficient DNA fingerprinting of crop cultivars and individuals is frequently used in both theoretical population genetics and in practical breeding. Numerous DNA marker technologies exist and the ratio of speed, cost and accuracy are of importance. Therefore even in species where highly accurate and polymorphic marker systems are available, such as microsatellite SSR (simple sequence repeats), also alternative methods may be of interest. Thanks to their high abundance and ubiquity, temporary mobile retrotransposable elements come into recent focus. Their properties, such as genome wide distribution and well-defined origin of individual insertions by descent, predetermine them for use as molecular markers. In this study, several Ty3-gypsy type retrotransposons have been developed and adopted for the inter-retrotransposon amplified polymorphism (IRAP) method, which is suitable for fast and efficient pea cultivar fingerprinting. The method can easily distinguish even between genetically closely related pea cultivars and provide high polymorphic information content (PIC) in a single PCR analysis.
Rice is a model genome for cereal research, providing important information about genome structure and evolution. Retrotransposons are common components of grass genomes, showing activity at transcription, translation and integration levels. Their abundance and ability to transpose make them good potential markers. In this study, we used 2 multilocus PCR-based techniques that detect retrotransposon integration events in the genome: IRAP (inter-retrotransposon amplified polymorphism) and REMAP (retrotransposon-microsatellite amplified polymorphism). Markers derived from Tos17, a copia-like endogenous retrotransposon of rice, were used to identify genetic similarity among 51 rice cultivars (Oryza sativa L.). Genetic similarity analysis was performed by means of the Dice coefficient, and dendrograms were developed by using the average linkage distance method. A cophenetic correlation coefficient was also calculated. The clustering techniques revealed a good adjustment between matrices, with correlation coefficients of 0.74 and 0.80, or lower (0.21) but still significant, between IRAP and REMAP-based techniques. Consistent clusters were found for Japanese genotypes, while a subgroup clustered the irrigated Brazilian genotypes.
Employing FISH analysis as well as BLAST and CUSTAL W (1.82) programs, we investigated types of DNA nucleotide sequences building an additional heterochromatic band in 2R chromosomes of 3 lines of Secale vavilovii Grossh. The probes used in FISH analysis were designed based on the reverse transcriptase sequence of Ty1-copia and Ty3-gypsy retrotransposons and the 5S rRNA gene sequence. No hybridization signals from the reverse transcriptase probes were observed in the chromosome region where the additional band occurs. On the other hand, signals were observed after hybridization with the 5S rDNA probe, clearly suggesting the presence of that type of sequences in the analyzed heterochromatin band. Using BLAST and CUSTAL W programs, we revealed high similarity of the JNK1 sequence to the 5S rRNA gene from Hordeum chilense (HCH1016, HCH1018, 88%) and to a fragment of the 5S rRNA sequence of H. marinum (HMAR003, 97%). In addition, the same fragment of JNK1 was shown to be very similar to the part of the Angela retrotransposon (92%) as well as to the SNAC 426K20-1 transposon (89%) belonging to CACTA family, both from Triticum monococcum, and to Zingeria biebersteiniana pericentromeric sequences (78%). The similarity of JNK1 to those sequences may be accidental or the JNK1 may represent an ancient mobile genetic element that caught the 5S rRNA sequence. During the evolution those sequences might have been accumulated in the particular region on the 2R chromosome. Our results suggest that the additional heterochromatin band in chromosomes 2R of S. vavilovii is a collection of defective genes and/or mobile genetic elements.
Retroviral envelope (env)-like sequences in 2 cultivated allotetraploid cottons and their diploid progenitors have been identified and characterized in this study. DNA sequence analysis reveals that these sequences are heterogeneous. The observed sequence diversity, however, seems to preserve coding information. This is evidenced by the detection of the transmembrane domain (TM), which is the most conserved feature of the divergent retroviral env genes. The high ratio of synonymous to nonsynonymous changes suggests that these sequences are evolving under purifying selection. Phylogenetic analysis shows that Gossypium sequences closely cluster with a lineage of plant endogenous retroviruses that have an env-like gene. These results provide evidence for the antiquity and the wide diversity of env-like sequences in the Gossypium genome.
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