Chromosomal organization of simple sequence repeats in the Pacific oyster (Crassostrea gigas): (GGAT)(4), (GT)(7) and (TA)(10) chromosome patterns.

Chromosome identification is essential in oyster genomic research. Fluorescence in situ hybridization (FISH) offers new opportunities for the identification of oyster chromosomes. It has been used to locate satellite DNAs, telomeres or ribosomal DNA sequences. However, regarding chromosome identification, no study has been conducted with simple sequence repeats (SSRs). FISH was used to probe the physical organization of three particular SSRs, (GGAT)(4), (GT)(7) and (TA)(10) onto metaphase chromosomes of the Pacific oyster, Crassostrea gigas. Hybridization signals were observed in all the SSR probes, but the distribution and intensity of signals varied according to the oligonucleotide repeat. The intercalary, centromeric and telomeric bands were observed along the chromosomes, and for each particular repeat every chromosome pair presented a similar pattern, allowing karyotypic analysis with all the SSRs tested. Our study is the first in mollusks to show the application of SSR in situ hybridization for chromosome identification and karyotyping. This technique can be a useful tool for oyster comparative studies and to understand genome organization in different oyster taxa.

Chromosomal evolution and phylogenetic analyses in Tayassu pecari and Pecari tajacu (Tayassuidae): tales from constitutive heterochromatin.

The mammalian family Tayassuidae (peccaries) is confined to the New World and comprises three recognized extant species, white-lipped (Tayassu pecari), collared (Pecari tajacu) and chacoan (Catagonus wagneri) peccaries, which exhibit distinct morphological and chromosomal features. The phylogenetic relationships among the tayassuids are unclear and have instigated debate over the palaeontological, cytogenetic and molecular aspects. Constitutive heterochromatin analysis can be used in understanding the phylogenetic relationships between related species. Here we describe, for the first time, the constitutive heterochromatin (C-positive heterochromatin) of two tayassuid species, Tayassu pecari and Pecari tajacu. We demonstrate that in situ restriction endonuclease digestion with sequential C-banding could be a complementary tool in the study of constitutive heterochromatin heterogeneity in chromosomes of the Tayassuidae. Our characterization of peccary chromosomes suggests that the Pecari tajacu autosomal karyotype is more primitive and has accumulated great diversity in its constitutive heterochromatin. This idea is supported by several other studies that analysed nuclear and mitochondrial sequences of the living peccary species. Finally, the tayassuid X chromosome primitive form seems to be the one of Tayassu pecari.

Morphological, yield, cytological and molecular characterization of a bread wheat x tritordeum F1 hybrid.

The morphological, yield, cytological and molecular characteristics of bread wheat x tritordeum F(1) hybrids (2n = 6x = 42; AABBDH(ch)) and their parents were analysed. Morphologically, these hybrids resembled the wheat parent. They were slightly bigger than both parents, had more spikelets per spike, and tillered more profusely. The hybrids are self-fertile but a reduction of average values of yield parameters was observed. For the cytological approach we used a double-target fluorescence in situ hybridization performed with total genomic DNA from Hordeum chilense L. and the ribosomal sequence pTa71. This technique allowed us to confirm the hybrid nature and to analyse chromosome pairing in this material. Our results showed that the expected complete homologous pairing (14 bivalents plus 14 univalents) was only observed in 9.59% of the pollen mother cells (PMCs) analysed. Some PMCs presented autosyndetic pairing of H(ch) and A, B or D chromosomes. The average number of univalents was higher in the wheat genome (6.8) than in the H(ch) genome (5.4). The maximum number of univalents per PMC was 20. We only observed wheat multivalents (one per PMC) but the frequency of trivalents (0.08) was higher than that of quadrivalents (0.058). We amplified 50 RAPD bands polymorphic between the F(1) hybrid and one of its parents, and 31 ISSR polymorphic bands. Both sets of markers proved to be reliable for DNA fingerprinting. The complementary use of morphological and yield analysis, molecular cytogenetic techniques and molecular markers allowed a more accurate evaluation and characterization of the hybrids analysed here.