Recent advances in cytology

A note on orientation in mitosis and meiosis

The most detailed and diverse data available from classical and especially allelic crossing over are examined on the basis of a hybrid DNA model of crossing over. The model is similar to that proposed byHolliday. There are three main points of difference: (1) It is proposed that crossing over begins with a switch of chromatids, as in the classical hypothesis. Dissociation of DNA then begins from the nearest recombinator and extends as far as the switch. The switch, therefore, limits the spread of hybrid DNA formation. (2) Since dissociation begins from the nearest recombinator, the midpoint between two recombinators is the point about which the probability of dissociation, either from the left or the right, is determined. (3) The probability of breaks in the chains that dissociated is higher near the recombinators, and the probability of breaks of the chains that did not dissociate is higher near the mid-point. The overall ratio of the two types of break is, however, 50∶50. The observations the model attempts to explain include the relationship between chiasmata and crossing over; positive interference; negative interference; polarity; differential rates of conversion.

Interpreting crossing over data.

The diploid chromosome number (2n = 2x = 16) in three wild species A. litigiosum Pau, A. pul- verulenthum Lazaro and A. subbaeticum Guemes, Sanchez & Mateu, belonging to the genus Antirrhinum was cytologically observed. Genetic stability of the simultaneous meiotic division type was found in these natural growing snapdragons. The synchronicity at the initial meiotic stages and at tetrad and pollen creation was high and disappeared with the diakinesis initiation. Refl ecting the systematic position of the studied species into Antirrhinum, the performed investigation mani- fested similarity: in the presence of univalents and quadrivalent confi gurations parallel with bivalents at the early meiotic stages; in the type of disturbances at metaphase and anaphase stages; in the tetrad formation and pollen characteristics; in the particularity of the tapetal cell division. There were dissimilarities in the bivalent chiasma formation and localisation and in the level of the meiotic dis- orders. Seven different chromosome associations were observed at diakinesis and metaphase one in A. pulveru- lenthum PMCs, while in the other two Antirrhinum species they ranged between 12 and 13. The highest value of ring bivalents, total and mean chiasma number per bivalent, was found in A. pulverulenthum PMCs, while the lowest value of these characteristics was evaluated in the A. litigiosum cells. The chromosome behaviour during anaphase stages showed more changes in A. subbaeticum PMCs and the most regularity in the A. pul- verulenthum cells, which were refl ected in the value of the pollen fertility (83.7% and 98.4% respectively). The established specify and differences demonstrated genetic diversity between A. litigiosum, A. pulverulenthum and A. subbaeticum genomes.

http://www.caryologia.unifi.it/past_volumes/63_3/11.pdf

Microsporogenesis in three wild species of the genus Antirrhinum L. (A. litigiosum Pau, A. pulverulenthum Lazaro and A. subbaeticum Güemes, Sánchez and Mateu)

IN two samples of Allium neapolitanum bulbs, received from Van Tubergen, Holland, Levan1 reported 14 and 28 chromosomes. Feinbrun2 also reported the tetraploid form (2n = 28) of this species growing wild in Palestine. In a sample I have received from de Jager's, Holland, I have found only the diploid. In a second sample, however, there were again tetraploids; in addition there were two new types, triploids (2n = 21) and pentaploids (2n = 35).

Allium neapolitanum: a Mixed Species

The derivation of the theory of “cyto-nuclear harmony” is outlined as substantiated by the following evidence.A. The evidence of known hybrids.B. The evidence of species indicating a hybrid ancestry.C. The evidence of diploid parthenogenesis.D. The evidence of chromosome duplication.E. The evidence of special Gases including…, 1. Haploid parthenogenesis.2. Gametophytic generations.A number of otherwise common but insignificant observations are shown to take an importance in view of the theory of “cytonuclear harmony” and yield profitable lins of investigation for a better understanding of the morphology of the dividing cell.

Recent advances in cytology

Citations

A note on orientation in mitosis and meiosis

Interpreting crossing over data.

Microsporogenesis in three wild species of the genus Antirrhinum L. (A. litigiosum Pau, A. pulverulenthum Lazaro and A. subbaeticum Güemes, Sánchez and Mateu)

Allium neapolitanum: a Mixed Species

The Significance of Meiotic Irregularities in Hybrids. The Theory of Cyto-nuclear Harmony

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