Variation in plastid inheritance between pelargonium cultivars and their hybrids

Abstract

The genetic control of plastid inheritance is analysed in cultivars and hybrids of zonal pelargoniums (Pelargonium x Hortorum Bailey). After crosses between maternal parents with normal green (G) plastids and paternal parents with mutant white (W) plastids in their germ cells, the progeny consist of a mixture of green, variegated and white embryos corresponding to a maternal, biparental or paternal inheritance of plastids in each individual. Families of individual embryos from selfed or crossed parents are of two patterns that are highly dependent upon the genotype of the female parent. Type I females produce families in which green embryos are most, variegated intermediate and white least frequent. Type II females produce families in which green and white embryos are of approximately the same frequency and variegated embryos the least frequent. Five type I and six type II cultivars and several hybrids between them were selfed, and 88 crosses of various kinds were made to create a heterogeneous population of 2601 plants that were tested for their plastid inheritance pattern by crossing with a standard male parent as source of mutant plastid; 1298 families were classified as type I and 1303 as type II. The maternal (G + V), biparental (V) and paternal (V + W) plastid percentage transmission was estimated for each family, and the frequencies of percentage classes within the population of type I or type II families expressed as histograms. The type I families fell into a skewed distribution with such a maternal bias that about one-half inherited only maternal plastids, and over two-thirds of them had at least some maternal plastids in every embryo. The average family had a maternal percentage of 98·7 per cent and paternal of 4·7 per cent, and no family was below 80 per cent maternal or above 25 per cent paternal. The overall maternal: paternal ratio was about 21:1. The type II families were more normally distributed with a large variance. The average family had a maternal percentage of 63·2 per cent and paternal of 38·1 per cent with an overall maternal: paternal ratio of about 5:3. On average 46·6 per cent of type I families and 64·7 per cent of type II families had biparental progeny. The frequencies of families decreased sharply with increasing proportions of biparental progeny. Analysis of variance of a subset of the type II population showed that there was a significant added variance component for maternal percentage, and a significant linear regression of offspring on parents; hence, in addition to the major gene difference determining type I versus type II patterns, there was a significant polygenic component causing variations within each pattern. Alternative hypotheses for the mechanisms determining differences in the pattern of plastid inheritance in Pelargonium, Oenothera, and Medicago are discussed. © The Genetical Society of Great Britain.