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Allozyme diversity in the tetraploid endemic Thymus loscosii (Lamiaceae)

Lopez-Pujol, J., Bosch, M., Simon, J., Blanche, C.
Annals of botany 2004 v.93 no.3 pp. 323-332
Thymus (Lamiaceae), indigenous species, endangered species, genetic polymorphism, autopolyploidy, allozymes, genetic variation, gene frequency, gene flow, population genetics, habitat fragmentation, Spain
Background and Aims: Thymus loscosii (Lamiaceae) is a tetraploid perennial species endemic to the Ebro river basin (north-eastern Spain), which is included in the National Catalogue of Endangered Species. It is a tetraploid species (2n = 54), presumably an autotetraploid originated by the duplication of a 2n = 28 genome and the subsequent loss of two chromosomes. Allozyme electrophoresis was conducted to survey the levels and distribution of genetic diversity and to test the previous autopolyploid hypothesis for its origin. In addition, both in situ and ex situ conservation measures are proposed. Methods: Eight populations were sampled for analysis by standard methods of starch gel electrophoresis, and six putative enzymatic loci were resolved (five consistently and one only partially). Key Results: Banding patterns exhibited no evidence of fixed heterozygosity and showed both balanced and unbalanced heterozygotes. In addition, most individuals showed a pattern consistent with the presence of three or four alleles at a single locus. High levels of genetic variability were found at population level (P = 85 %, A = 3.0, He = 0.422), in addition to a trend of an excess of heterozygotes. Conclusions: Allozyme data support the hypothesis that T. loscosii is an autotetraploid, and the high number of alleles at some loci may be due to repeated polyploidization events. The high values of genetic variation found in this species agree with those expected for tetraploids. The excess of heterozygotes may be due to some barriers to inbreeding (e.g. occurrence of gynodioecy) and/or selection for heterozygosity.