Llavallol, Argentina
Instituto Fitotécnico de Santa Catalina (FCAF, UNLP)
Centro de Investigaciones Genéticas (UNLP-CONICET-CIC)
Meiotic analysis of the artificial hybrids between Zea luxurians and the subspecies of Zea mays. --Gonzalez G*, Comas, C*, Naranjo, CA, Poggio, L*

*also affiliated with Dpto. de Ciencias Biológicas, FCEN, UBA.

Meiotic behavior in Zea luxurians and three subspecies of Zea mays belonging to Section Zea (Z. mays ssp. parviglumis, Z. mays ssp. mexicana, Z. mays ssp. mays) and their artificial F1 hybrids have been analyzed to investigate their meiotic configurations and to detect chromosomal rearrangements acting as reproductive isolation mechanisms. Poggio et al. (Genome 49:993-1000, 1999) showed that the hybrids between Zea luxurians and the other species of the Sect. Luxuriantes (Z. perennis and Z. diploperennis) have irregular meiotic configurations and low fertility.

The F1 hybrid Z. luxurians x Z. mays ssp. parviglumis showed pachytene heterozygosis in the knob regions (Fig. 1A), meiotic asynchrony of two groups of 5 II each (diplotene-diakinesis) (Fig. 1B), heteromorphic bivalents and univalents of different size (diplotene-diakinesis, metaphase I) (Fig. 1C), bridges and fragments (anaphase I), and laggard chromosomes (anaphase I and II). These phenomena were also observed in the F1 hybrid Z. luxurians x Z. mays ssp. mexicana (Fig. 1D, E, F)). In the F1 hybrid Z. luxurians x Z. mays ssp. mays heteromorphic bivalents and 1-2 bridges and fragments (anaphase I) were observed (Fig. 1 G-K).

In these hybrids 8 II + 4 I was the most frequent configuration, although the range of univalents were 0-20 per cell (Fig. 1D). Moreover, they were highly sterile (pollen stainability: 0-12%) and showed loss of pairing in heterozygotes for knobs in pachytene. The genome size varies between 2C= 5.8 - 6.8 pg in the subspecies of Zea mays, while Z. luxurians has 2C= 9 pg. The difference in the genome size between the hybridís parental species could explain the presence of heteromorphic bivalents. The presence of two asynchronic groups of 5 II was also frequent.

The bridges and fragments, observed in anaphase I, in the studied hybrids would indicate that the parental species differ in 2-3 paracentric inversions between them.

The presence of two groups of 5 II suggests that the genomes of these hybrids maintain structural and functional independent domains, despite the existence of intergenomic rearrangements revealed by molecular data.

Figure 1. A-C: Z. luxurians x Z. mays ssp. parviglumis. D-F: Z. luxurians x Z. mays ssp. mexicana. G-K: Z. luxurians x Z. mays ssp. mays.

A: Pachytene: the arrows show irregular pairing in the knob regions.
B: Diakinesis: meiotic asynchrony of two groups of 5 II each.
C. Metaphase I: the arrow shows a heteromorphic bivalent, univalents of different size are observed off the plate.
D: Diakinesis: 20 univalents.
E: Metaphase I: heteromorphic bivalent and univalents of different size off the plate.
F: Anaphase I: Two bridges and two fragments.
G: Diakinesis: 10 heteromorphic bivalents.
H: Metaphase I: heteromorphic bivalent.
I - J: Open heteromorphic bivalents.
K: Anaphase I showing one bridge.

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