CHIASMATA IN A PERICENTRIC INVERSION IN ZEA MAYS

Abstract

NVERSIONS can provide critical material for testing the validity of the partial I chiasmatype hypothesis. According to this hypothesis crossing over within an inversion should lead to the formation of several characteristic bivalent configurations in diplotene and diakinesis. As pointed out by DARLINGTON (1937) and recently by BROWN and ZOHARY (1955), some of these configurations, namely inversion asymmetry and the open reversed inversion chiasma, could offer, if found, critical indication that crossing over actually leads to formation of a chiasma. Such inversion configurations have been observed by BROWN and ZOHARY (1955) to occur regularly in microsporocytes of Lilium formosanum heterozygous for a long paracentric inversion. For confirmatory evidence it was felt desirable to look for such configurations in other organisms. The present paper reports the results of such a search in a case of a long pericentric inversion in chromosome 1 of maize. MATERIALS 4 N D METHODS Several seed of Zea mays, obtained from selfing a plant heterozygous for a peri-centric inversion in chromosome l (points of breakage: lS.3-1L.5), were grown at Berkeley in the summer of 1954. A plant heterozygous for this inversion was selected from this group after identification of the inversion loop at pachynema. Spikelets from this plant were fixed in 3:l alcohol-acetic acid for 24 hours. Anthers were smeared in aceto-carmine and temporary slides were used for examination and photography, one to four days after their preparation. Throughout this study, microsporocytes have been first observed under low dry magnification. Only these cells in which all the ten bivalents could be seen to be well separated from one another were chosen for further observation. OBSERVATIONS Microsporocytes in mid-diakinesis were found to be the most suitable material for the present study. In addition, observations have been made also in early diplotene. Late diplotene and early diakinesis stages could not be utilized, since cells in these stages showed considerable stickiness and clumping of chromosomes together. Metaphase I bivalents were too compact for reliable identification of chiasmata. Early diplotene Several inversion bivalents have been observed a t this stage in which the pachytene inversion loop (or part of it) was still arrested (fig. 1 81 2). The bivalent photographed in figure 2 can perhaps best be interpreted as having two chiasmata within the inversion. However, the general absence of strong repulsion at this stage in regions ' Present atldress: