Genetic Control of Apospory in Apomictic Paspalum Species

Abstract

The genetic control of aposporous apomixis in Paspalum was investigated by determination of the reproductive mode of the progeny from a cross of the sexual, self-incompatible tetraploid, P. ionanthum by the facultative apomict, P. cromyorrhizon. One hundred and sixty-nine progeny were assessed using embryological analysis, of these, 127 were shown to produce aposporous embryo sacs whilst 42 were completely free giving a ratio of 3.02/1. Fourteen plants of the aposporous group showed a high level of female sterility and may be considered as being members of the non-aposporous group which would alter the ratio to 2.02/1. Selfing of the female parent produced 120 caryopses of which only 25 gave rise to flowering plants. Six of the latter showed occasional aposporous embryo sac formation. Cytological analyses of meiosis indicates that any interpretation of the genetic ratios must take into account disomic or tetrasomic inheritance in P. ionanthum and tetrasomic with random chromatid assortment in P. cromyorrhizon. On this basis the observed phenotypic ratios can be accommodated within a genetic model of the sexual female parent being of the genotype AAAa if tetrasomy applies or Aa if disomy, whilst the apomitic male is Aaaa. In this situation apospory requires the presence of at least two doses of the recessive allele together with a single copy of the wild type allele, homozygous recessive genotypes will be non-viable. Alternatively, a model of one dominant gene controlling apospory together with lethality of gametes carrying two copies of the gene may account for the results.