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Keywords:

  • Female frequency;
  • gynodioecy;
  • heterozygosity;
  • population structure;
  • Thymus vulgaris L

Abstract

In a self-compatible gynodioecious species, the abundance of female plants (which are obligate outcrossers) relative to hermaphrodites (which may self and outcross) may be a critical factor influencing genetic diversity and population structure. In the gynodioecious Thymus vulgaris L., female frequency varies from 5 to 95%, providing a suitable model to examine this issue. In this study, we use allozyme markers to (1) evaluate the relationship between female frequency, genetic diversity and population structure, (2) determine whether females and hermaphrodites vary in heterozygote deficiency and (3) examine whether other factors such as plant density are related to heterozygote deficiency. Twenty three natural populations, with female frequencies ranging from 11 to 92%, were sampled in and around the St-Martin-de-Londres basin in southern France. Based on four polymorphic allozyme loci, we found no significant correlation between female frequency and heterozygote deficiency. A significant (P < 0.05) FIS value over loci and over populations of 0.11 was detected. The FIS value per population showed a significant heterozygote deficiency in 11 of the 23 populations. However, no significant difference between female and hermaphrodite FIS values was found. A significant heterozygote deficiency only occurred in populations of intermediate density. There was little differentiation among populations (FST = 0.038) nor among subpopulations within each population. The significant FIS values are thus mostly due to inbreeding effects. The lack of a correlation between FIS values and female frequency may be due to outcrossing in hermaphrodites and/or restoration of male fertility which may occur to a greater extent at low female frequency. The similarity of female and hermaphrodite FIS values indicates that females may occasion high levels of biparental inbreeding.