ISO-chromosomes and the origin of triploidy in hybrids between old and new world species of Philadelphus

Abstract

1. AllPhiladelphus species in nature are diploids 2n=26. 2. The chromosome complement of species examined was found to be fairly uniform in morphology and could be represented by the haploid formulae. 1L_m+8M_st+2S_st+2S_sm3. The L chromosome inPhiladelphus was found to be an iso-chromosome with a weak centromere. 4. While geographical isolation inPhiladelphus has not appreciably affected pairing between the 12 pairs of chromosomes in hybrids, it has a distinct effect on the pairing of iso-chromosomes. 5. Pollen fertility of species and hybrids is related to the behaviour of iso-chromosome. 6. Triploidy in interspecific hybrids is due to the formation of unreduced gametes by non-separation of iso-chromosomes at meiosis.