In situ hybridization, a powerful tool for the molecular cytogenetlcist, can be used to physically map repetitive, low-copy, and unique DNA sequences in plant chromosomes. With the availability of a recombination map in Oryza sativa L. and an improved in situ hybridization technique, this study was designed to esablh the relationship between the genetic and physical distances of the rice restriction agment length polymorphism map. Analysis indicated that considerable variation can exist between genetic and physical maps. A 183-centimorgan linkage map for chromosome 2 covered <50% of the chromosome and did not include the centromere, whereas a 91-centmorgan linkage map for chromosome 1 covered '80% of the chromosome. The results indicated that there are potential "hot" and "cold" spots of recombination and polymorphisms in rice, which involve both genes and restriction frgment length polymorphisms.

The scanning electron microscope may be useful in the analysis of plant chromosomes treated with in situ hybridization, especially when the probes and (or) chromosomes are near or beyond the resolution of the light microscope. Usual methods of plant chromosome preparation are unsuitable for scanning electron microscope observation as a result of cellular debris, which also interferes with probe hybridization. A method is described whereby protoplasts are obtained from fixed root tips by enzymatic digestion and applied to slides in a manner that produces little or no cellular debris overlying the chromosomes. The slides were examined by scanning electron microscopy and light microscopy after C-banding and in situ hybridization with a rye nucleolus organizer region spacer probe. This technique, which allows for scanning electron microscope visualization of bands and probes not easily identified with light microscopy, should prove useful in the physical mapping of low copy number or unique DNA sequences.Key words: protoplasts, rice, wheat, rye, physical maps, in situ hybridization.

Crosses between oeioploid and hexaploid ti'iticales have been made in breeding programs for several T.-ears. From an analysis of the progen)

The influenee of telomerie heteroehromatin on early embryo and endosperm development and the produetion of aberrant endosperm nuelei were investigated using eight tritieale (X Tritocosecale Wittniaek) lines homozygous for the presenee or absenee of blocks of telomerie heteroehromatin from rye ehrotnosome arms lRL, 2RS, 6RS, and 7RF. The loss of heteroehromatin from two ehromosomes (6R, 7R) produeed a signifieantly lower percentage of aberrant endo.spenn nuelei than did the loss from one ehromosome (6R). Deletion of a third bloek had a significant additional effect in only one of four lines. The differenees between lines for rates of embryo and endosperm development were significant even within a karyotype, presumably beeause of genetie variation. The onset of eellularization of the endosperm tended to obseure the deteetion of aberrancy, and sinee it evidentty oeeurred at different times in different lines, may have been largely responsible for the apparent reduetion in aberrant nuclei in lines with less heteroehromatin.

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