Indeterminate Meiotic Restitution (IMR): a novel type of meiotic nuclear restitution mechanism detected in interspecific lily hybrids by GISH

A detailed analysis of microsporogenesis was carried out in three diploid l ily cultivars (2n=2x=24) and three diploid interspecific hybrids (2n=2x=24) using DNA in situ hybridisation methods (GISH and FISH). In cvs. Gelria (L ilium longiflorum; L genome), Connecticut King and Mont Blanc (both Asiatic hybrids; A genome) meiosis was regular and only haploid gametes were, form ed while the three interspecific hybrids between L. longiflorumxAsiatic hyb rid (LA) showed a variable frequency of meiotic nuclear restitution and sta inable 2n-pollen formation ranging from 3% to 30%. An analysis of meiotic c hromosome behaviour of the LA hybrids through GISH and FISH revealed that: (1) the parental chromosomes could be clearly discriminated into univalents , half-bivalents and bivalents in the PMCs; (2) in some of the PMCs the ent ire complement was present either as univalents or half-bivalents which had the potential to divide equationally (following centromere division) durin g the first division leading to first division restitution (FDR) gametes; ( 3) more frequently, however, in one and the same PMC the univalents and hal f-bivalents divided equationally whereas the bivalents disjoined reductiona lly at the same time giving rise to 2n-gametes that could vary from the wel l-known FDR or SDR 2n-gametes. We indicate this novel type of restitution m echanism as Indeterminate Meiotic Restitution (IMR). In order to confirm th e occurrence of IMR gametes, the chromosome constitutions of eight triploid BC, progenies derived from backcrossing the 2n-gamete producing the LA hyb rids to the Asiatic hybrid parents were analysed through in situ hybridisat ion. The results indicated that there were seven BC, plants in which FDR 2n -gametes, with or without homoeologous recombinations, were functional, whe reas in one case the 2n-gamete resulting from IMR was functional. In the la tter, there was evidence for the occurrence of genetic recombination throug h homoeologous crossing-over as well as through the assortment of homoeolog ous chromosomes. A singular feature of the IMR 2n-gamete was that although it transmitted a euploid number of 24 chromosomes to the BC1 progeny, the n umber of chromosomes transmitted from the two parental species was dissimil ar: 9 L-genome chromosomes and 15 A-genome chromosomes instead of 12 of eac h.