Annually recurring parthenogenesis in a zebra shark Stegostoma fasciatum.

A zebra shark, Stegostoma fasciatum, held in captivity at the Burj Al Arab aquarium, produced embryos and pups in the absence of a male. A total of 15 pups were produced from eggs laid within the aquarium over a period of four consecutive years commencing 2007. Parthenogenesis was confirmed through DNA analysis for three pups sampled during the first two consecutive egg cycles and is presumed to be the method of reproduction responsible thereafter.

A 4,103 marker integrated physical and comparative map of the horse genome.

A comprehensive second-generation whole genome radiation hybrid (RH II), cytogenetic and comparative map of the horse genome (2n = 64) has been developed using the 5000rad horse x hamster radiation hybrid panel and fluorescence in situ hybridization (FISH). The map contains 4,103 markers (3,816 RH; 1,144 FISH) assigned to all 31 pairs of autosomes and the X chromosome. The RH maps of individual chromosomes are anchored and oriented using 857 cytogenetic markers. The overall resolution of the map is one marker per 775 kilobase pairs (kb), which represents a more than five-fold improvement over the first-generation map. The RH II incorporates 920 markers shared jointly with the two recently reported meiotic maps. Consequently the two maps were aligned with the RH II maps of individual autosomes and the X chromosome. Additionally, a comparative map of the horse genome was generated by connecting 1,904 loci on the horse map with genome sequences available for eight diverse vertebrates to highlight regions of evolutionarily conserved syntenies, linkages, and chromosomal breakpoints. The integrated map thus obtained presents the most comprehensive information on the physical and comparative organization of the equine genome and will assist future assemblies of whole genome BAC fingerprint maps and the genome sequence. It will also serve as a tool to identify genes governing health, disease and performance traits in horses and assist us in understanding the evolution of the equine genome in relation to other species.

EXO1 and MSH4 differentially affect crossing-over and segregation.

The 5'-3' exonuclease Exo1p from Saccharomyces cerevisiae is required for wild-type levels of meiotic crossing-over and normal meiotic chromosome segregation as is the meiosis-specific MutS homologue, Msh4p. Mutations in both genes reduce crossing-over by approximately two-fold, but deltamsh4 strains have significantly lower viability and a higher frequency of meiosis I non-disjunction. Epistasis analysis indicates a complex interaction between the two genes. Although crossing-over was not detectably lower in the double mutant, viability was significantly worse than either single mutant. Such a result suggests that the two genes are affecting meiotic viability by distinct mechanisms. We propose that deltaexo1 affects chromosome segregation by reducing crossing-over, while deltamsh4 affects both the frequency and distribution of crossovers. Mutation in EXO1 reduces gene conversion frequencies significantly at some but not all loci, suggesting that other enzymes are also involved in DNA resection. We propose that Exo1p plays an early role in establishing some recombination intermediates by generating single-stranded tails. The role of Msh4p is suggested to be in determining whether some recombination intermediates are resolved as crossover events and in generating crossover interference. The synergistic effect of deltaexo1deltamsh4 on spore viability suggests that the two genes have partially compensatory roles in a process affecting meiotic success.

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.

Meiosis in the leek (Allium porrum L.) revisited. I. Prophase I pairing.

Meiotic chromosome pairing of the tetraploid species Allium porrum, the cultivated leek, was analysed by electron microscopy of 83 surface-spread nuclei in the late zygotene to early diplotene interval of prophase I, from four different varieties. Prophase I quadrivalent frequency, at 71%, marginally but significantly exceeds the two-thirds expected on a simple random end-pairing model, suggesting that more than two autonomous pairing sites occur, in some tetrasomes at least. This pattern of synaptic behaviour is consistent with an autotetraploid status, but comparison with other tetraploids, including other Allium species, indicates that Allium porrum may be a weak segmental allopolyploid displaying limited preferential homologous pairing. The incidence of pairing partner switches (PPSs) in prophase I quadrivalents is relatively low; 90% of all analysed quadrivalents had only one or two PPSs. The positional distribution of PPSs along quadrivalents was distinctly uneven with peaks in mid-chromosome arms and reduced frequencies around centromeres and near the ends. The four different varieties of leek analysed were remarkably similar in their meiotic behaviour despite their diverse breeding history, but individual plants within varieties displayed substantial variation in quadrivalent and PPS frequencies.