The Effect of Chromosome Structure upon Meiotic Homologous and Homoeologous Recombinations in Triticeae

The tribe Triticeae contains about 500 diploid and polyploid taxa, among which are important crops, such as wheat, barley and rye. The phylogenetic relationships, genome compo-sition and chromosomal architecture, were already reported in the pioneer genetic studies on these species, given their implications in breeding-related programs. Hexaploid wheat, driven by its high capacity to develop cytogenetic stocks, has always been at the forefront of these studies. Cytogenetic stocks have been widely used in the identification of homoeologous relationships between the chromosomes of wheat and related species, which has provided valuable information on genome evolution with implications in the transfer of useful agronomical traits into crops. Meiotic recombination is non-randomly distributed in the Triticeae species, and crossovers are formed in the distal half of the chromosomes. Also of interest for crops improvement is the possibility of being able to modulate the intraspecific and interspecific recombination landscape to increase its frequency in crossover-poor regions. Structural changes may help in this task. In fact, chromosome truncation increases the recombination frequency in the adjacent intercalary region. However, structural changes also have a negative effect upon recombination. Gross chromosome rearrangements produced in the evolution usually suppress meiotic recombination between non-syntenic homoeologs. Thus, the chromosome structural organization of related genomes is of great interest in designing strategies of the introgression of useful genes into crops.

The karyotype of Alouatta fusca clamitans from Rio de Janeiro, Brazil: Evidence for a y-autosome translocation

The chromosome complements of four males of Alouatta fusca clamitans, caught in Rio de Janeiro State, Brazil, were analyzed by G-, C-, and NOR-banding techniques. The diploid number found was 49 in all the specimens. The presence of a heteromorphic pair of submetacentric chromosomes in the analyzed specimens, not present in males and females with 2n = 50 previously reported, and its G-banding pattern, led us to assume that this pair is involved in a Y-autosome translocation. Thus, the sex determination system appears modified to X1X1X2X2 /X1X2Y. Heterochromatic segments were found in the pericentromeric region of all the chromosomes, in the telomeric region of the short arm in pair 2, in the complete length of the short arm of pairs 5 and 6 and in the intercalary region of the long arm in pair 17. The nucleolar organizer regions were situated in the intercalary region of the long arm in two small acrocentric pairs.

Chromosome banding homologies in three species of Aedes (Stegomyia)

The chromosome complements of the mosquitoes Aedes aegypti, Aedes mascarensis, and Aedes albopictus, belonging to the subgenus Stegomyia, gave a uniform response to the Q-, H-, and R-banding techniques. Of the three homomorphic chromosome pairs, only the shortest or sex pair (I) showed a consistent banding pattern. In the three species, a bright yellow intercalary band was present on one arm of both chromosomes of the sex pair after heat treatment and staining with acridine orange. The rest of the chromosome and the other two pairs fluoresced orange–red. The same intercalary region appeared completely dark with the fluorochromes quinacrine and Hoechst 33258, while the rest of the chromosomes fluoresced dull. The same banding pattern was present in males and females. Size variations of the Q- and H-negative and R-positive intercalary bands were observed within each species. The results are interpreted in terms of structural homology of the sex-determining chromosomes, which is retained within the subgenus.Key words: sex-determining chromosomes, banding (Q, H, R), Aedes, mosquitoes.

The composition of the cell-wall at the apical meristem of stem and root

It is a striking feature of the growth of any highly organised plant body that the construction of new protoplasm and consequent formation of new cells is usually strictly localised to certain definite regions, known generally, as the meristematic tissues. In the normal flowering plant, the main meristematic regions of the axis are found at the apices of stem and root as the apical or polar meristems and distributed in the intercalary region as two thin cylinders of cambial meristem, one between xylem and phloem, the vascular cambium, the other the cork phellogen, situated near the periphery. No tissues are more important in plant development than these meristematic regions, but so far their study has mainly been carried out by cytological methods, which have supplied much information as to the structural organisation of the protoplast, and especially of the nucleus. In the present paper, two of these meristematic regions, namely, the polar meristems of shoots and roots, are studied with reference only to the biochemical changes that proceed within the wall separating the protoplasts. Originally these walls are extremely thin, and from general considerations, as well as from cytological observations upon the phenomena at the completion of anaphase, it would appear that these walls, commencing as interfaces in a protein-containing medium, may be regarded as composed at first mainly of protein. The original wall may be homogeneous in physical structure, but will be of extremely complex chemical nature. From the observations that follow it would appear that its subsequent history represents chemically a progressive simplification; the constituent substances segregate into special lamellæ as they are released, so that the change is accompanied by an increasing complexity of organisation, of which the distinction between middle lamella and inner wall is the first visible indication.