Chromosome end arrangement in Datura ceratocaula.

IT HAS BEEN the purpose of this investigation to examine the segmental arrangement of the chromosomes of Datura ceratocaula in relation to the segmental arrangement of the chromosomes of the standard Datura stramonium (prime type 1), and to study the relationship of the chromosomes of Datura ceratocaula to the chromosomes of the other herbaceous species of Datura. The determination of these chromosomal relationships together with data on the geographical distribution and external morphology of the species in question may afford information about the evolution of the herbaceous species of Datura. The examination of the chromosome end arrangement was accomplished by obtaining hybrids of Datura ceratocaula with various prime types (PT's) of Datura stramonium (Bergner et al., 1933) and with the other species of this genus and observing the number of bivalents and the configurations formed at the first meiotic metaphase in these hybrids. MATERIALS AND METHODS.-D. ceratocaula is an aberrant species of Datura which is native to Mexico where it grows in shallow ponds. It differs from the other herbaceous Datura species in that it has a hollow stem from the lower portion of which roots may be produced. The plant of this species used in the present investigation was S.I. 2952. In order to determine the segmental arrangement of the chromosomes of D. ceratocaula it was necessary to obtain hybrids between D. ceratocaula and the standard prime type 1 of D. stramonium and also with other prime types of this species. In addition, hybrids were obtained between D. ceratocaula and eight other herbaceous species of this genus in order to compare the chromosomal end arrangements. No viable seeds have ever been obtained from numerous pollinations between D. ceratocaula and the other species of Datura but by using the embryo dissection technique, McLean (1946) was able to obtain these hybrids for the first time. The technique used for the dissection of hybrid embryos of Datura was that originally described by van Overbeek et al. (1942). The time from pollination of the female parent to the production of flower buds in the hybrid offspring varied from 4-7 months. An attempt was made to relate the metaphase stage of meiosis to the size of the bud so that fewer buds would have 1 Received for publication March 18, 1948. Contribution from the Department of Botany, Smith College, New Series No. 28. The article is taken from a thesis submitted to the faculty of Smith College in partial fulfillment of the requirements for the degree of Doctor of Philosophy under the direction of Dr. A. F. Blakeslee of the Smith College Genetics Experiment Station. 2S.I. is an abbreviation for seed introduction number. Each new group of seeds received is given an S.I. number which all its progeny retain. to be examined for meiotic figures. It was found that the size of the bud, when the chromosomes were in the first meiotic metaphase, varied from hybrid to hybrid and also varied with certain environmental differences, light, temperature, etc. In general, a certain size range could be determined for each hybrid which corresponded approximately to the stage required, as shown in table 1.