AlIozyme Uniformity Within and Variation Between Races of the Colonizing Species Xanthium Strumarium L. (Noogoora Burr)

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Abstract
The degree of polymorphism at 13 enzymic loci was tested in 12 populations of Xanthium strumarium representing all four naturalized races of this species in Australia. The races are X. chinense, X. italicum, X. pennsylvanicum and X. cavanillesii. Very little variation was found within races of X. strumarium at the loci studied. In marked contrast to this lack of intraracial variation there was considerable interracial genetic differentiation at several loci. It is concluded that genetic polymorphisms and hence populational buffering, at least at the enzyme level, has been of little significance in the adaptation of this species during its colonization of Australian environments. The observed outcrossing rate in X. italicum was zero (inestimable in the other races) with an upper confidence limit of 10-12 %, confirming previous observations that X. strumarium is predominately self-pollinated.

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