Meiotic synapsis of the Allium porrum B chromosome: evidence for a derived isochromosome origin

Ultrastructural analysis of B chromosome synapsis in surface-spread (2B) pollen mother cells of the leek, Allium porrum, has clarified their structural organization and shed new light on their origin. In pachytene cells containing two B chromosomes, these chromosomes either formed a pair of univalents showing foldback hairpin loops or synapsed together to form bivalents of several different types. The synaptic configurations of univalents and bivalents indicate that these B chromosomes have a basically isochromosome organization, but this is modified by a slight centric shift giving an arm ratio of 1.1:1. This analysis adds to the growing number of B chromosomes that have been shown to be isochromosomes or isochromosome derivatives. Key words : Allium porrum, B chromosomes, synapsis, synaptonemal complex, isochromosome.

Synapsis in grasshopper bivalents heterozygous for centric shifts

Analysis of surface-spread synaptonemal complexes of zygotene and pachytene spermatocytes was carried out on centric-shift heterozygotes of grasshoppers. These rearrangements affected the M7 chromosome in Chorthippus vagans and the M6 and S8 chromosomes in Chorthippus apricarius. The shifts in the latter two chromosomes were also associated with C-heterochromatin variations between homologous chromosomes. Rearranged chromosomes proceeded directly to heterosynapsis without an apparent intervening homosynaptic phase in M7 bivalents of Ch. vagans and M6 bivalents of Ch. apricarius. In the latter case, axial equalization of the heterochromatin polymorphism was also achieved. On the other hand, asynapsis of the intercentromeric regions throughout pachytene was the rule in the centric shift involving the S8 chromosome of Ch. apricarius. In the three cases analysed, the production of unbalanced gametes in the heterozygotes is precluded either by the lack of chiasma formation in heterosynapsed rearranged segments or by the lack of pairing between such segments. Chiasmata were limited to the homologous regions of the heteromorphic bivalents.Key words: synapsis, surface spreading, centric shift, chiasma distribution, meiosis.

Genomic instability in a clonal species, Tradescantia commelinoides (Commelinaceae)

Within a population of Tradescantia commelinoides collected in the wild, genomic instability is unique to clones having large centric shifts. Initially, the instability is attributable to breakage caused by imperfect separation of abnormally long chromatids, but the more severe effects that sometimes develop are a consequence of spontaneous breakage and reunion and disturbance of control mechanisms. Breakage of excessively long chromatids following repositioning of the centromere may destabilize some segment of DNA important for ordered replication and division. Eventual stability would result from the loss of this segment, or its transfer to a quiescent position. A period of instability, leading to the establishment of a new equilibrium, may represent an important source of variability in species that rely heavily upon vegetative reproduction for their colonizing potential.Key words: Tradescantia, clonal species, centric shift, genomic instability, karyotype orthoselection.