CONICET | Buscador de Institutos y Recursos Humanos

Gene rearrengements, genome downsizing and gene silencing are phenomena said to accompany the polyploidization process. In order to detect if some of these modifications have occurred to the rDNA loci of S. elaeagnifolium (an aggressive weed with 2n = 24, 48 and 72 populations growing spontaneously in Argentina), the FISH technique was applied. The probes employed were the pTa71 containing the 18S-5.8S-26S gene of wheat, and a 5S rDNA probe obtained by PCR with DNA of S. stuckertii. The results showed that number of rDNA signals and ploidy level are correlated: the diploids presented a pair of signals for the 18S-5.8S-26S and pair for the 5S, while tetra and hexaploids increase their FISH marks proportionally to the ploidy level. In all cases, rDNA loci were asyntenic. These facts could suggest that the polyploid series is recently formed and drastic rearrengements have not yet happened. However, the 4x and 6x individuals presented differences in the intensity of their 18S-5.8S-26S signals: in each cell, a pair of marks was evidently stronger than the rest. This could indicate that some loss of gene copies might be taking places at these loci. In other hand, one hexaploid individual showed seven 5S signals instead of the expected six. The extra signal, in heterozygosis, indicate that this individual could have one parent with a duplicated 5S locus. At the same time, it implies more than one place of origin for the hexaploids, a matter that has been previously suggested by studies on geographical patterns and DNA content of S. elaeagnifolium. In S. elaeagnifolium, the pattern of variation of rDNA loci number with polyploidy does not fit exactly with the findings in other genera, as it does not always correspond either to the exact proportionality or to a relative loss of sites.

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