The Structure and Meiotic Behavior of the Differentiated Chromosomes of Tomato.

OR a number of years the elongate chromosomes of salivary gland nuclei F of Diptera and the pachytene chromosomes of plants have proved of use in researches in cytogenetics. The pachytene and later meiotic chromosomes of the tomato offer interesting material for cytological and perhaps cytogenetic studies because of their morphology. The chromosomes of the tomato are strikingly differentiated into regions differing in diameter and staining capacity. I n addition, the differential regions seem to show differential behavior during meiosis with respect to pairing, location of chiasmata, and contraction. The arms of the tomato chromosomes are divided into two sorts of regions. One sort is adjacent to the centromere, is relatively broader, and stains very deeply with iron aceto-carmine. Because of its staining capacity, this sort of region will be called a chromatic zone. The other sort forms the distal portion of the chromosome arms, is relatively narrower, and stains very lightly. This sort of region will be called an achromatic zone. The chromatic zones have a very distinct pattern of specific chromomeres while the achromatic regions possess very delicate chromomeres which are difficult to observe. The transitions from the chromatic to the achromatic zones are very abrupt. The achromatic regions all terminate in small, but very distinct k n o h , or telochromomeres which thus form the ends of the chromosomes. The chromosomes of the tomato seem to correspond most closely in structure to the meiotic chromosomes of Agapanthus (BELLING 1928; DARLINGTON 1933; GEITLER 1933), Oenothera (JAPHA 1939; MARQUARDT 1937), Pe lia (JACHIMSKY 1935), and Sphaerocarpus (LORBEER 1934). Each of the features mentioned above for the tomato has been reported for one or more of these plants. I t is possible to see all of these characteristics clearly in the tomato. Because of its importance as an economic plant, the tomato has been the subject of numerous cytological investigations. Although achromatic zones have been recognized a t pachytene (LESLEY and LESLEY 1935) and achromatic threads a t diakinesis pictured by many workers, all previous investigators have concerned themselves largely with the chromatic material. Technical difficulties have apparently prevented previous workers from obtaining a clear picture of chromosome structure in the tomato. Some of these difficulties have recently been overcome by the development of a mordanting modification of the aceto-carmine smear technique by DR. MARTA S. WALTERS. Even with this method which gives an unusually intense stain to the chromatic zones, the