Role of fluorescence in situ hybridization in sequencing the tomato genome.

The tomato (Solanum lycopersicum L.) genome is being sequenced by a consortium of laboratories in 10 countries. Seventy-seven percent of the tomato genome (DNA) is located in repeat-rich, gene-poor, pericentric heterochromatin, while 23% of the genome is located in repeat-poor, gene-rich, distal euchromatin. It is estimated that approximately 90% of tomato's nuclear genes can be characterized by limiting the sequencing effort to euchromatin while avoiding the problems involved in sequencing the repetitive DNA in heterochromatin. Sequencing is being performed on tomato nuclear DNA cloned into bacterial artificial chromosome (BAC) vectors. Fluorescence in situ hybridization (FISH) is used to help direct the sequencing effort by cytologically demonstrating the location of selected BACs on tomato chromosomes. While mitotic metaphase chromosomes are too short and compact for this purpose, long pachytene chromosomes are ideal. BACs localized in euchromatin can be used confidently as anchors for the assembly of BAC contigs that extend through the euchromatic length of each chromosome arm. Another important role for FISH is identification of BACs near telomeres and near borders with pericentric heterochromatin to indicate that sequencing should not extend much further. This role of FISH is enhanced by our ability to estimate base pair distances between localized BACs and these chromosomal features. Finally, it is noteworthy that when BAC-FISH is combined with chromosomal in situ suppression (CISS) hybridization to block repeats and localize single/low copy sequences, the great majority of BACs localize to single sites. This observation is consistent with tomato being an ancient diploid.

Distributions of percentage methaemoglobin in several population groups in California.

Several epidemiological studies in California have yielded data on percentage methaemoglobin in healthy individuals. The population groups represented include infants, elementary and high school children, and adults. The distributions of values in each group are described, as well as the differences between groups. Factors affecting the distributions are discussed. In the study of infants, the factors assessed include respiratory and gastrointestinal disease, and food and water intake. In schoolchildren, the effect of age and location of residence within Southern California are evaluated, and in adults, smoking, gender and time of day. Among infants in the 31 to 60 day age range, 33 per cent had methaemoglobin levels of 3 per cent or above, while 8 per cent had methaemoglobin levels of 4 per cent or above. Among adults, 15 per cent had levels of 3 per cent or above, while 2 per cent had levels of 4 per cent or above. Among both elementary and high school students, 3 per cent had methaemoglobin levels of 3 per cent or above, while less than 1 per cent had levels of 4 per cent or greater.