Genetic Variation among South American Populations of Tetraploid Chenopodium sect. Chenopodium subsect. Cellulata

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Abstract
Electrophoretic analysis of South American Chenopodium (sect. Chenopodium subsect. Cellulata, 2n = 4x = 36) reveals multi-locus isozyme systems for leucine aminopeptidase (LAP) and phosphoglucoisomerase (PGI) that appear to be the result of tetraploid gene duplication. The bi-locus LAP system is monomorphic for active alleles with putative null alleles at each locus that are not present in populations at the northern range extreme. The null allele at Lap-1 appears to be sporadically distributed in the highlands of Peru, Bolivia, and Argentina, whereas the null variant at Lap-2 shows a clinal increase from north to south with fixation in the Argentine lowlands. All material (62 population samples from Chile, Argentina, Bolivia, and Peru, and 206 exemplar samples from Ecuador, Peru, and Bolivia) is monomorphic at Pgi-1 and essentially monomorphic at the other two PGI loci, with the exception of low elevation populations of Argentina, which show five active variants and one null. Aside from clinal trends at two loci, the observed pattern of genetic variation is not consistent with extreme discontinuities in selective regime, habitat, and distribution. It is proposed that an archaic genotype has been conserved in the Andean area of chenopod cultivation through interpopulation gene flow mediated by weed-crop genetic interaction and human dispersal. A relatively high level of inter-population reproductive isolation in lowland Argentina has possibly allowed differentiation of PGI alleles and silencing of one LAP locus through fixation of null alleles. Populations from lowland Argentina include individuals showing electrophoretic diploidization in both isozyme systems, thus producing the typical phenotype of diploid Chenopodium. The percentage of heterozygous plants per population and level of detected out-crossing indicate a high potential for gene flow within populations of lowland Argentina, although genetic interaction with sympatric C. album was not detected.