Nucleotide polymorphism analysis of the Asr2 gene in wild tomato populations dwelling in arid habitat

GIOMBINI M; FRANKEL N; HASSON ER; IUSEM N

Foz do Iguaçu

Congreso; XII Congreso latinoamericano de Genetica; 2006

The Asr2 gene is a member of the Asr (Abscisic acid[ABA]-Stress-Ripenning) gene family which is present in Spermatophyta. Asr2 is up-regulated under water stress conditions (Iusem et al., 1993) and is suspected of playing an important role in water deficit tolerance through ABA signalling pathway (Rossi and Iusem, 1994). In a previous work (Frankel et al., 2003), Ka/Ks ratios were calculated comparing Asr2 sequence from different species within Lycopersicon genus (tomatoes). These results suggested that this gene might have been a target of natural selection in tomato species dwelling in dry areas. However, molecular intraspecific variation was not extensively evaluated in the species surveyed by Frankel et al. (2003). Intraspecific polymorphism analysis could shed light on evolutionary forces and demographic process that colud have affected Asr2 history. Preliminary observations of Asr2 nucleotide variation in Lycopersicon chilense (accession LA2884), which inhabits a desertic region of Chile, yielded a complete lack of polymorphism as expected for those loci that experienced a selective sweep. On the other hand, a few alleles sampled from Lycopersicon peruvianum v. humifusum (accession LA0385), a population localized in a semiarid region of Peru, exhibited a considerable number of segregating sites including several nonsynonymous variants. Ruling out other possibilities, balancing selection might account for such nucleotide variation pattern.We sampled 17 and 14 Asr2 alleles from LA2884 and LA0385, respectively; and 11 alleles from another population of Lycopersicon peruvianum v. humifusum (accession LA2151) situated nearby LA0385. In addition we sampled another non-linked locus (CT114) to evaluate potential demographic events that might have affected the whole genome. After obtaining the nucleotide sequences, polymorphism and genealogic structures were analyzed. Asr2 showed almost no nucleotide variation in LA2884 as expected from previous data. Neutrality tests based on polymorphism spectrum frequency (Tajima’s D, Fu y Li’s D’ and Fu’s F) gave results consistent with a selective sweep. Evidences from other study (Städler et al., 2005) indicated that LA2884 population could have suffered a bottleneck. Therefore, a population size decrease could also explain the low Asr2 nucleotide variation found in LA2884.The presence of higher levels of polymorphism in Asr2 within LA0385, as suggested by preliminary observations, was confirmed by our results. We found two well differentiated haplotype groups in the Asr2 allelic genealogy as expected under balancing selection. Interestingly, Asr2 alleles from LA2151 do not differ from those belonging to one of the LA0385 haplotype group. This observation indicates either that LA2151 is the result of a recent founder event by individuals from LA0385, or that unidirectional gene flow has existed from LA2151 toward LA0385. The latter scenario is an alternative explanation to the balancing selection hypothesis that may account for the elevated level of polymorphism and haplotype structure observed in LA0385