ALBERT

Notes: Summary and conclusions A small derivative of the standard type of B chromosome in rye was definitely demonstrated to be an iso-chromosome, (i) because both arms have the same chromomere structure, (ii) because the two arms of the same chromosome frequently pair with each other and (iii) because the four arms of a bivalent at pachytene may exchange partners with each other, forming a pseudo-tetravalent. The small iso-fragment is derived from the standard fragment because the arms have a chromomere structure identical to that of the short arm of the standard fragment. This identity implies (i) that the three main chromosome regions of the short arm of the standard fragment (cf.Lima-de-Faria 1952) are also present in both arms of the small iso-fragment, (ii) that the size and distribution of the chromomeres is the same and (iii) that the cone shaped terminal part of the short arm constituted by three chromomeres may be distinguished in both the mother chromosome and its derivative, the small iso-fragment. Owing to a high degree of meiotic elimination the frequency of transmission of the small iso-fragment is low even in plants with 2 such chromosomes.

Notes: Summary The seven chromosome pairs of the normal chromosome complement ofSecale céréale (A chromosomes) have been identified at pachytene. A detailed chromomere analysis of the A chromosomes and the “standard fragment” (a type of B chromosome previously described) including the number, size, and sequence of the chromomeres, permitted the construction of eight reference maps of these chromosomes. (1) There is a good agreement between the chromomere number and chromosome length of the different A chromosomes and the standard fragment. (2) Knob formations are seriations of large chromomeres and deeply stained fibrillae. The 11 knob formations of the A chromosomes plus the one of the standard fragment are all accompanied on one or both sides by seriations of particularly small chromomeres. The organization of the kinetochoric structure is of the same type in all the seven A chromosomes and the standard fragment. (3) Small conspicuous seriations composed generally of two consecutive pairs of larger chromomeres having between them a fibrilla pair particularly weakly stained are found at regular intervals in the chromosome arms. This results in a regional differentiation of the chromosome. (4) The length of the thick regions adjacent to the kinetochores is apparently not correlated with arm length. When the length of the arm varies it is mostly the median regions — with medium size chromomeres — that vary in length. (5) Chromomere pairs having chromomeres of unequal size or having one of the chromomere partners absent are found in all chromosomes. This evidence of the structural heterozygosity of the chromosome material of rye is in agreement with the cross-fertilizing breeding system of the species. (6) A structural rearrangement involving from one to seven chromomere pairs is present in chromosome VI. The number of chromomeres involved in this rearrangement varies from cell to cell, the rearrangement being absent in some P.M.C.'s. (7) Not all chromomeres have the usual round shape but some possess a hairy structure. Hairy chromomeres appear with a much greater frequency in the positively heteropycnotic regions of the chromosome where chromomeres are large than in the other regions of the chromosome where the chromomeres are of medium size or small. (8) It could be ascertained that the standard fragment of rye — like that in maize — is evolved to such an extent that there is no segment of the A chromosomes that can be directly homologized with it. B chromosomes may have arisen from A chromosomes through a hereditary variation in the properties of the kinetochore of an A chromosome, or their process of origin may have been accompanied by such a hereditary variation. (9) The standard fragment is not an exception to the general rule of the heterochromatic constitution of the B's since it also shows a certain degree of heteropycnosis when compared with the A chromosomes at pachytene. Both the number and extension of positively and negatively heteropycnotic regions is bigger in the standard fragment than in the A chromosomes. (10) The average distance in micra between the centres of two consecutive chromomeres decreases with increasing size of the chromomeres. At the same time the distance between the centres of large chromomeres is approximately of the same length as their diameter. (11) All A chromosomes possess the same general chromomeric pattern with minor variants. In all A chromosomes the chromomeres are particularly large on both sides of the kinetochore. There is a subsequent general decrease in chromomere size on both sides of the kinetochore towards the chromosome ends, where an abrupt increase in chromomere size takes place in most cases (knob formations). This gradient of chromomere size originates on both sides of the kinetochore where chromomeres of large size are found and finishes in chromomeres of very small size at the chromosome ends or just behind the knob formations when these are present. The regulation of the rate at which this decrease in chromomere size takes place is apparently controlled in the first place by the knob formations and, in their absence, by the arm length. The existence of this gradient and the variation of its shape in relation to knob formation position and arm length indicate that the size of a chromomere is not only determined by its genetical constitution and nuclear environment but is also dependent on its position within the chromosome. (12) Chromosomes III and VI are rather similar to chromosomes IV and VII respectively. The existence of a chromomere size gradient in the arms of the A chromosomes of rye furnishes the basis for an interpretation that conciliates at the same time the apparent genetical differentiation of these chromosomes with their morphological similarity. (13) In rye pachytene chromosomes there is no essential difference, at the mircoscopic level, between the structural organization of euchromatic and heterochromatic regions. Heteropycnosis appears as a phenomenon determined not only by the genetic constitution and nuclear environment of a certain chromosome region but also by the position of this same chromosome region within the chromosome body. (14) The structural comparative analysis of on one hand, the part of the chromosome arms not including the knob formations, and on the other hand kinetochores and knob formations, reveals that in rye pachytene chromosomes there is no essential morphological difference, at the microscopic level, between the components of the kinetochores, those of knob formations, and those of the remaining part of the chromosome arms, all these three chromosome regions being composed of chromomeres and fibrillae which are morphologically identical. This analysis also furnishes a series of data that permit an understanding of the apparently deviating morphological appearance of kinetochores and knob formations. The pachytene chromosome appears, at the microscopic level, as a structural unity.

Notes: Summary (1) In the chromosomes ofAgapanthus umbellatus the kinetochore can be seen to be divided as early as pro-metaphase II. At this stage, the two longitudinal halves of the kinetochore are individualized and separated from each other. The region of the chromosome body that appears to be responsible for holding together the two sister chromatids of each chromosome until the beginning of anaphase II is the most proximal region of the arms and not the kinetochore. (2) At the second division of meiosis, three regions with distinct cycles of division may be distinguished in the chromosomes of this species: (1) the kinetochore which can be seen to be divided from prometaphase II, (2) the most proximal regions of the arms which only divide at the beginning of anaphase II and (3) the rest of the chromosome arms which appears to be divided from early prophase II. The concept of division is revised. (3) The kinetochore has its own cycle of division but does not appear to take direct part in holding together the two sister chromatids of each chromosome until the beginning of anaphase II. The fact that this last property appears to be situated in the arms has several implications. The behaviour of t-ends and of the regions of the B chromosome types responsible for the phenomenon of non-disjunction can be better understood in the light of this finding. In addition, the particular type of division of the mitotic chromosomes ofUlophysema öresundense can be more easily interpreted.