The pairing of chromosomes in the F1 crossB. juncea Coss. (2n = 36) XB. campestris L. var.sarson Prain (2n = 20) takes place according to the Drosera scheme. With the exception of one cell, which showed 1iv+9II + 6I, all the rest showed 10II + 8I. From this and other evidence the amphidiploid origin ofB. juncea as a hybrid betweenB. campestris andB. nigra has been proved.
In the crossB. Tournefortii Gouan (2n = 20) xB. trilocularis H.f.T. (2n = 20) the pairing of chromosomes is very variable. Nuclei with total lack of pairing to those showing a quadrivalent besides 1–3 bivalents were observed. The whole meiotic process in the hybrid is very irregular, resulting in the formation of dyads, monads and tryads to the extent of 25, 8 and 5%, respectively.
The conjugation of chromosomes in the hybridB. trilocularis H.f.T. (2n = 20) xB. rapa L. (2n = 20) is complete, 10II being invariably formed at diakinesis and metaphase I. Non-disjunction of a bivalent was frequently observed at anaphase I. But for this irregularity the whole meiotic process is quite regular.
B. sinapistrum Boiss. has 2n = 18,B. Tournefortii Gouan 2n = 20,B. monensis Huds. 2n=24 andB. rugosa 2n=38 chromosomes.
Somatic chromosomes of nine species were examined, with special reference to the number of satellites and muceloli. A complete correspondence between the two has been found.
WhereasB. nigra Koch, which has the lowest chromosome number in the genus (2n=16) shows four satellites and four nucleoli,B. oleracea L. (2n = 18),B. rapa L. (2n = 20),B. campestris L. (2n = 20),B. trilocularis (2n = 20) andB. Tournefortii (2n = 2) show only one pair each. This suggests that the latter five species have lost the extra pair of satellites by mutation during the course of evolution. The presence of six satellites and six nucleoli inB. juncea and four nucleoli inB. napus and.B. rugosa has been correlated with their amphidiploid nature, and is additional proof of this.
Other interesting features in somatic mitosis, such as somatic pairing, sporadic aberrations in chromosome numbers and formation of chromatin bridges, have been described.
From the secondary association of chromosomes in three species the primary basic number for the genus has been inferred to be five. This conclusion is supported by the chromosome numbers of some related genera in which the basic number five has been retained for polyploidy. In certain other genera the hexaploid number thirty is basic, and some species have become tetraploid on this basis. Some other evidence has also been adduced to show that all the monogenomic species ofBrassica have a common origin.
WhileB. campeslris, B. trilocularis, B. Tournefortii andB. rapa show exclusively bivalent formation at meiosis,B. juncea, B. sinapistrum andB. monensis show occasionally a quadrivalent.B. nigra andB. Wrightii show in addition higher multivalents. The probable reasons for the formation of multivalents in each of this species have been discussed.
Presence of relatively inverted segments of chromosomes has Journ. of Genetics Sl been inferred in some species from the chromatin bridges formed at meiosis.
The role played by gene mutations, structurai changes of chromosomes and hybridization in species formation inBrassica has been discussed. Of these hybridization has played the most important part, as amphidiploidy has frequently occurred in the genus.