Subterminal Centromeres Research Articles

KARYOTYPES OF IRIS PUMILA AND RELATED SPECIES

Randolph, L. F. and Jyotirmay Mitra. (Cornell U., Ithaca.) Karyotypes of Iris pumila and related species. Amer. Jour. Bot. 46(2): 93‐102. Illus. 1959.—The karyotypes of 30‐ and 32‐chromosome geographical variants of the amphidiploid I. pumila from Russia and the Balkans were compared with the karyotype of the typical 32‐chromosome Austrian forms of this species and with those of the diploid I. attica and I. pseudopumila, previously reported to be the basic species from which I. pumila originated. Plants from 3 collections of a Crimean form of I. pumila with 32 chromosomes had a pair of long chromosomes with submedian centromeres morphologically similar to chromosome 1 of the typical form of I. pumila. In addition, there was another heteromorphic pair of submedian chromosomes with one of the members having a shorter short arm. The manner in which this altered chromosome could have arisen as a result of a heterobrachial inversion is described. Five different collections of I. pumila with 30 chromosomes from Russia differ in several respects from the typical 32‐chromosome I. pumila. They have an unusually long pair of chromosomes with a submedian centromere and a secondary constriction in the long arm. This chromosome is the original chromosome 2 which had been altered by the addition of a segment equivalent to the most of the long arm of one of the shorter chromosomes with subterminal centromere. The manner in which this could have occurred as the result of unequal reciprocal translocation is described. Loss of the remaining diminutive portion of the short chromosome with subterminal constriction assumed to have been involved in the unequal interchange of segments producing the modified, longer chromosome 2 would account for the reduction in chromosome number from 32 to 30 in the Russian form of I. pumila. Four pairs of chromosomes with satellites have been found in the 30‐chromosome plants whereas 6 pairs of satellited chromosomes are present in the 32‐chromosome I. pumila. The spontaneous occurrence of chromosomal alterations of the type here described are considered to be significant factors in the process of chromosomal repatterning resulting in the appearance of new geographical races, and eventually of species of iris, with altered chromosome numbers and modified karyotypes. More specifically it is concluded that amphidiploidy accompanied by chromosomal repatterning resulting from segmental interchange, heterobrachial inversion and related types of chromosomal alterations has played an important role in the evolution of I. pumila and karyological forms of this species occupying different geographical areas.

A Reinvestigation of the Cytology ofCampelia Zanonia(L.) H. B. K.

SUMMARY1. Campelia zanonia, a monotypic taxon of Tradescantiae-Hexandrae, has 2n = 16 chromosomes. The karyotype consists of two pairs of long chromosomes with submedian centromeres, four pairs of medium-sized ones with subterminal centromeres among which one pair has the satellites, and two pairs of short chromosomes with subterminal centromeres.2. Neither multivalents nor chains of bivalents have been observed during meiosis. Therefore it is neither a polyploid nor a case of segmental Interchange as was previously described. It is just a normal diploid.

CYTOTAXONOMIC STUDIES ON EREMOPYRUM

Five species are included in the genus Eremopyrum, namely, E. triticeum, E. hirsutum, E. buonapartis, E. orientale, and E. distans. Of these, the first two are diploids with 2n = 14, while the rest are tetraploids with 2n = 28. The karyotypes of the two diploid species are different from those of any other member, of the Triticineae in that they have chromosomes with extremely subterminal centromeres. The karyotype of each of the three tetraploid species shows a mixture of two types of chromosomes: seven pairs with extreme subterminal centromeres and the other seven pairs with median or submedian centromeres. This indicates that these are amphidiploids between triticeum–hirsutum-like taxa and perhaps some diploid species of crested Agropyron. In view of the karyotypic peculiarity, annual habit, and reported sterility barrier, the decision to give Eremopyrum generic status is supported.

A supernumerary chromosome in Locusta.

THE chromosomes of Locusta migratoria normally comprise eleven pairs of autosomes ranging from 1.2µ to 5.3µ, with two X-chromosomes in the female and one X in the male1. In 1934, however, Itoh2 described locusts in a Japanese population which contained an additional chromosome, present in variable numbers and supernumerary to the normal complement. Recently, we have found such supernumerary chromosomes in a stock used in this Department. Each extra chromosome has a sub-terminal centromere and is slightly larger than any member of the three smaller autosome pairs. In these and other respects the supernumeraries appear to be similar to those found by Itoh in Japan. Dr. C. E. Ford and Mr. J. L. Hamerton, of the Radiobiological Research Unit, Harwell, and Mr. C. G. Elliott, of the Botany School, Cambridge, have kindly informed us that they too find supernumerary chromosomes in locust stocks related to our own.

THE COMPOUND X CHROMOSOMES IN DROSOPHILA

There are four ways in which two X chromosomes may be joined to give a compound with two ends. The centromere of the compound may be either median or terminal and, in each case, the two component chromosomes may be in reversed order with respect to each other or they may be arranged in tandem. A simple self-descriptive terminology is achieved if the compounds with median centromeres are referred to as metacentrics and those with terminal or subterminal centromeres as acrocentrics, and if these terms are preceded by tandem or reversed, depending on the relative order oi the two components. The four possibilities are, then, the tandem metacentric, the tandem acrocentric, the reversed metacentric and the reversed acrocentric. It should be noted that the attached X chromosome is the reversed metacentric compound ; the latter name is introduced here not to supplant the well established attached X designation, but to indicate the structural relation of the attached X to the other types. Finally, there are two possible compounds in which the two X chromosomes are joined to form a continuous ring. Here also the two components may be in reversed or tandem order. The first is the reversed ring, the second, the tandem ring. ORIGIN OF THE COMPOUND x CHROMOSOMES

Heteroploidy in mammals

1. Forty-two spontaneously heteroploid eggs have been found in the mouse (Mus musculus). 2. Haploid, triploid, tetraploid, hexaploid, diplo/polyploid mosaic, aneuploid and multiform aneuploid eggs have been identified. The commonest form of heteroploidy is triploidy. Up to three heteroploid eggs have been observed in the same female. 3. All but one of the heteroploid eggs were found in a particular laboratory stock (silver stock) which is associated with thesilver factor. 4. In any one mouse the triploid eggs contain on the average fewer nuclei than diploid eggs. 5. Heteroploid eggs are capable of apparently normal development up to the blastula stage, the latest stage at which they have so far been examined. The possibility of further development is being investigated. 6. The silver factor, or a closely linked factor, is probably responsible for the production of heteroploid eggs in silver stock. 7. As in germ cells, chromosomes in the pre-implantation mouse egg are rod-shaped and most if not all chromosomes have a subterminal centromere. 8. The origin and frequency of occurrence of these heteroploid eggs are discussed, and also some consequences which might attend any further viability.

THE MORPHOLOGY OF SOMATIC CHROMOSOMES IN LILIUM

THE MORPHOLOGY OF SOMATIC CHROMOSOMES IN LILIUM

Memoirs: The Chromosomes of the Cymothoid Igopod Anilocra

Anilocra mediterranea has a diploid chromosome number of twelve, eight of the chromosomes having median, and four subterminal centromeres. The localization of chiasmata is extreme and is of a previously undescribed type, combining the terminal and centric systems. The centromeres stain differentially from the non-centric regions of the chromosomes; they are the only parts stained at anaphase of first meiosis, and at metaphase and anaphase of second meiosis.