Invasive Species Compendium

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Abstract

Cytological study on Mikania cordata (Asteraceae), a native plant in China.

Abstract

This study aimed to confirm the chromosome number of Mikania cordata (Asteraceae), a native plant in China, and to test the possible hybridization scenario between M. cordata and its invasive congener M. micrantha from a cytological perspective. Chromosome number and chromosome morphology of four populations in M. cordata were investigated. These included Hualian, Miaoli, Yilan and Taipei populations from Taiwan, China. The chromosome number 2n=36 was determined for all four populations. Their karyotypes, all formulated as 2n=18m+18sm, were characterized by having a remarkably larger, submedian centromeric chromosome pair with a secondary constriction in the middle of the long arms. All the karyotypes were Stebbins's 2B type. The intrachromosomal asymmetry index (A1) varied from 0.38 to 0.39, and the interchromosomal asymmetry index (A2) varied from 0.30 to 0.32. This is the only report of the chromosome number for M. cordata from China, and also the first karyotype report for this species. Our results, together with previous reports, indicate that there is aneuploid variation within this species, but 2n=36 (diploid based on x=18) is the only number currently known from the populations in Taiwan, China. The karyotypes of M. cordata resemble those reported for other Mikania species in the secondary constriction of the first chromosome pair, which could be considered a cytological marker for this genus. Mikania cordata and M. micrantha have the same chromosome number and ploidy but different karyotypic characteristics. However, intermediate karyotype was not found within populations where the two came into close contact. No intermediates between the two species were observed during field work, and ISSR analyses also failed to detected hybrid individuals. Therefore, the above evidence suggest that the successful invasion of M. micrantha in China is not associated with hybridization and introgression between this invasive species M. cordata and its only indigenous congener in China. After comparing past herbarium records with results of recent field surveys, we found that the distribution range of M. cordata in China has undergone a great reduction. We infer that habitat destruction and invasion of M. micrantha may be the main causes for the gradual disappearance of M. cordata in Taiwan, China.