Preliminary observations on Zea diploperennis and its hybrid with Zea mays

Meiosis in Zea diploperennis Iltis, Doebley and Guzman is normal. At diakinesis ten bivalents are formed. The pachytene chromosomes show normal synapsis and are terminated by prominent chromomeres which may be referred to as small knobs. There are 11 to 13 terminal knob positions. Although these positions are constant in subtending one or both arms, they have knob-like structures ranging in size from that of a prominent chromomere up to that of a more obvious knob. Similar observations have been recorded for the tetraploid perennial teosinte (A. E. Longley 1941). In this feature they differ from maize chromosomes which have internal knobs.

The total lengths of the ten chromosomes of diploperennis in comparison with maize are slightly longer, especially three of them, namely chromosomes 2, 3 and 7 where the difference is significant. This was confirmed in the hybrid material where the teosinte chromosomes could be seen extending beyond their maize homologue. The arm ratios are almost the same for both maize and diploperennis. A table comparing the total lengths and arm ratios from a total of about 400 cells observed at pachytene is given below.
 
Total Lengths
Arm Ratios
Maize* Teosinte Maize Teosinte
65.49 68.35 1.2 1.3
56.06 64.35 1.4 1.8
51.19 61.49 2.0 2.0
50.05 52.91 1.6 1.6
50.05 48.62 1.1 1.1
34.89 40.04 3.2 3.2
40.04 51.48 2.6 2.3
40.04 44.33 3.0 3.3
34.89 37.18 2.0 2.0
28.60 32.89 2.6 2.7
*(Rhoades, 1950)

In the hybrid between maize and diploperennis meiosis was normal. At diakinesis in 90% of the cells ten bivalents were formed. But occasionally microsporocytes with two univalents were also seen. Pollen fertility was normal.

In almost all the teosintes, inversions have been reported in one chromosome or the other. The chromosomes of this hybrid showed two heterozygous inversions, a paracentric inversion loop in the long arm of chromosome 5 and a terminal inversion loop in the short arm of chromosome 9. In some cells the terminal inversion in chromosome 9 did not form a loop, but the inverted segments remained unpaired. Though the inversions were observed there was no sign of bridges or fragments in the 400 cells studied so far. This could be due to failure of recombination in the region of the inversion or due to infrequency of their occurrence.

It is significant that the tetraploid perennial teosinte (Z. perennis) has the same chromosome 9 inversion, reported by Y. C. Ting, as that reported here for the diploid. Thus, most of the cytological evidence supports the contention that the tetraploid is an ancient autotetraploid of the diploid and therefore that the two taxa should be considered as a single species.

C. V. Pasupuleti and W. C. Galinat


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