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

GIOMBINI, M. I. ; FRANKEL, N.; HASSON, E. R.; IUSEM, N. D.

Foz do Iguaçu

Congreso; 52º CONGRESSO BRASILEIRO DE GENÉTICA y 12º CONGRESO DE LA ASOCIACIÓN LATINOAMERICANA DE GENÉTICA; 2006

Sociedade Brasileira de Genética y Asociación Latinoamericana de Genética

The Asr2 gene is a member of the Asr(Abscisic acid[ABA]-Stress-Ripenning) gene family which is present inSpermatophyta. Asr2 is up-regulated under water stress conditions (Iusem et al., 1993) and is suspected of playing an important rolein 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 Asr2sequence from different species within Lycopersicon genus (tomatoes).These results suggested that this gene might have been a target of naturalselection in tomato species dwelling in dry areas. However, molecularintraspecific variation was not extensively evaluated in the species surveyedby Frankel et al. (2003). Intraspecific polymorphism analysis could shedlight on evolutionary forces and demographic process that could have affected Asr2history.Preliminary observations of Asr2 nucleotidevariation in Lycopersicon chilense (accession LA2884), which inhabits adesertic region of Chile, yielded a complete lack of polymorphism as expectedfor those loci that experienced a selective sweep. On the other hand, a fewalleles sampled from Lycopersicon peruvianum v. humifusum (accessionLA0385), a population localized in a semiarid region of Peru, exhibited aconsiderable number of segregating sites including several nonsynonymousvariants. Ruling out other possibilities, balancing selection might account forsuch nucleotide variation pattern.We sampled 17 and 14 Asr2 alleles fromLA2884 and LA0385, respectively; and 11 alleles from another population of Lycopersiconperuvianum v. humifusum (accession LA2151) situated nearby LA0385. Inaddition we sampled another non-linked locus (CT114) to evaluatepotential demographic events that might have affected the whole genome. Afterobtaining the nucleotide sequences, polymorphism and genealogic structures wereanalyzed.Asr2 showed almost nonucleotide variation in LA2884 as expected from previous data. Neutrality testsbased on polymorphism spectrum frequency (Tajima's D, Fu y Li's D*and Fu's F) gave results consistent with a selective sweep. Evidencesfrom other study (Städler et al., 2005) indicated that LA2884 population couldhave suffered a bottleneck. Therefore, a population size decrease could alsoexplain the low Asr2 nucleotide variation found in LA2884.Thepresence of higher levels of polymorphism in Asr2 within LA0385, assuggested by preliminary observations, was confirmed by our results. We foundtwo well differentiated haplotype groups in the Asr2 allelic genealogyas expected under balancing selection. Interestingly, Asr2 alleles fromLA2151 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 founderevent by individuals from LA0385, or that unidirectional gene flow has existedfrom LA2151 toward LA0385. The latter scenario is an alternative explanation tothe balancing selection hypothesis that may account for the elevated level ofpolymorphism and haplotype structure observed in LA0385.