Cytogenetics of Rutaceae IV. Structure and systematic significance of interphase nuclei

Abstract

The chromatin of interphase nuclei in plants exhibits a very variable structure which depends on a series of nuclear and chromosomal parameters, the most important of which appear to be the amount and density of nuclear DNA and the nature and quantity of heterochromatin (Barlow 1977, Nagl 1982, Guerra 1985a). If we assume that these parameters are relatively constant within each species, especially in root-tip cell nuclei, we should expect the nuclear structure also to be constant and definite for each species. When viewed with the light microscope, the nucleus can be characterized on the basis of chromocenter number, size and shape and of the general appearance of non-chromocentric chromatin. Some chromatin remains condensed practically throughout the entire nuclear cycle, forming heterochromatic chromocenters or the heterochromatic portion of the chromo-centers. The remaining chromatin may have the appearance of a chromatic reticule of variable density in the interphase nucleus forming aggregations which constitute the condensed euchromatin (Nagl 1979). This chromatin fraction may be associated with heterochromatic chromo-centers or may form separate euchromatic chromocenters. Generally, the latter can be distinguished by more heterogeneous and less dense staining and by their irregular shape, frequently resembling projections of the heterochromatic chromocenters (Guerra 1985a). Several authors have established different classifications of types of interphase nuclei (Eichhorn 1934, Delay 1946–48, Tschermack-Woess 1963, Nagl and Fusenig 1979). In the present paper, we shall consider three fundamental types: areticulate, semi-reticulate and reticulate, according to the classification proposed by Delay (1946–48). The same author also considered a fourth type (“eureticule”) which was not found among Rutaceae. Considering that the nuclear structure, in contrast to other cytogenetical parameters such as DNA amount of chromosome number, cannot be quantified, and that the variation of the nuclear structure is very wide and continuous, the fundamental types must be divided into subtypes, as done by other investigators for large numbers of species and genera (Delay 1946–48, Tanaka 1971). In the present study we shall consider eight types of structural organization of interphase chromatin, although, if we wanted to be more rigorous in classifying each type, this number would certainly be higher. The main objective of the present study was to determine the potentiality of the structure of the interphase nucleus as a cytotaxonomic parameter for Rutaceae. At the same time, several observations were made on the origin and nature of chromocenters and on their relationship with the organization of prophase chromosomes. The results were interpreted in the light of previous information on chromosome number and size, DNA amounts and heterochromatin distribution patterns obtained for these species (Guerra 1984a, b, 1985b) which will permit better characterization of nuclear types. © 1987, Japan Mendel Society, International Society of Cytology. All rights reserved.