Chromosomal inversion heterozygosity in Paeonia intermedia (Paeoniaceae)

Meiosis of three populations (22 individuals) in Paeonia intermedia C.A. Mey. (Paeoniaceae) was investigated. Meiotic aberrations occurred so frequently that all individuals were found with no exception to be paracentric inversion heterozygotes, marked by chromosomal bridges and fragments at either anaphase I or II. However, the frequency of bridge/fragment occurrence and the size of fragments varied among individuals and among populations, showing that different inversions of various kinds existed in the species. Other aberrations, e.g., univalent, rod bivalent, laggard, anaphase I asymmetric segregation, were also observed. Mean rod bivalents per cell in Population One was 2.17, in Population Two was 2.04, and in Population Three was 2.21. A 0.21 of chromosomes on an average appeared as univalent in this species. The mean meiotic configurations were 2n = 10 = 0.25I + 4.87II in Population One, 2n = 10 = 0.20I + 4.90II in Population Two, and 2n = 10 = 0.17I + 4.92II in Population Three. Worse compatibility between the homologous chromosomes made the metaphase I pairing index to drop down to 69.50-81.07% among individuals. We propose that extensively existing inversion heterozygosity in the natural populations has a selective advantage, otherwise the heterozygotes has soon become homozygotes in the process of segregation of the progenies. However, the mechanism of how and why the species could maintain the heterozygosity in all the individuals of the wild populations, needs further study.