The genus Zea (Tribe Maydeae) is composed, according to Doebley and Iltis (Am. J. Bot. 67:994-1004, 1980), of two sections: Sect. Zea and Sect. Luxuriantes. The last section includes the perennials Z. diploperennis Iltis, Doebley & Guzman (2n=20) and Z. perennis (Hitch.) Reeves & Mangelsdorf (2n=40) and the annual Z. luxurians (Durue & Archenon) Bird (2n=20). In the last few.years, cytological evidence arose about the polyploid nature of all species of the genus Zea (Molina and Naranjo 1987, Theor. Appl. Genet. 73: 542-550; Poggio et al. 1990, Theor. Appl. Genet. 79:461-464; Naranjo et al. 1990, Acad. Nac. Cs. Ex. Fís. Nat., Buenos Aires, Monografía 5:43-53; Naranjo et al. 1994, Hereditas 120:241-244; Poggio and Naranjo 1995, Proceeding of lll Latin American and XVI Andean Zone of Maize Researches Meeting. Bolivia, Tomo ll:883-892). These authors postulated for the first time that five is the original basic number for the genus Zea. Some of the most suggestive cytological evidence which allowed postulating this hypothesis, stemmed from the analysis of meiotic behaviour of 2n=30 hybrids such as Z. perennis x Z. diploperennis and Z. perennis x Z. mays.
The present contribution deals with the analysis of the meiotic behaviour of the intrasectional hybrid Z. luxurians x Z. perennis (2n=30). These hybrids are highly sterile (pollen stainability <10%). The most frequent meiotic configuration was 5III + 5II + 5l (XIII = 5.17, ranging from 8 to 3; XII=4.65, ranging from 2 to 8; Xl=4.52, ranging from 2 to 7) (Table 1). The most parsimonious hypothesis to explain this result is to consider x=5 for the genus, as was postulated in previous reports from our laboratory.
Table 1. Meiotic configurations in the F1 Z. perennis x Z.
|Hybrid||2n||Meiotic Configurations||No. of cells|
|Z. perennis x Z. luxurians||30||8||2||2||4|
The meiotic configuration of the Z. luxurians x Z. diploperennis
(2n=20) hybrid was also analyzed. This hybrid is highly sterile and its
pollen stainability lower than 10%. The most frequent configuration observed
was 8II + 4l (XII=8.65, Xl=2.70). At least three of these bivalents are
very often heteromorphic.
The three species and both hybrids were also analysed by means of numerical techniques. With this purpose 13 vegetative morphological characters of plants and 10 reproductive ones (4 of prolificity, 4 from panicles and 2 from glumes) were used. The five OTU's (Operational Taxonomic Units) were grouped by means of UPGMA methods using the coefficient of mean taxonomic distance (DTM).
The phenogram constructed with these data shows that Z. diploperennis (Zd) and Z. perennis (Zp) are grouped, constituting the most proximal pairs of species. Then, both hybrids come out from this first group and finally, Z. luxurians (Zl) emerges at a greater distance with respect to the other OTU's.
In order to estimate the molecular genomic affinities among all three species of the section, these were also analysed by means of "dot blot" hybridization. Total genomic DNA from Z. perennis, Z. diploperennis and Z. luxurians was labelled with digoxigenin dUTP and used as a probe over dot blots on nylon membranes of similar serial concentrations of DNA of the same species. The technique was carried out according to Ann Kenton et al. (Evolutionary Paterns and Processes, Chapter 12: 190-206, The Linnean Soc. Press, 1993) with some minor modifications.
Table 2. Relating genomic affinities analysed by means of "DOT blot".
All probes were hybridized with all three species (Table 2), even with high stringency washes. However, Z. perennis and Z. diploperennis showed more molecular affinity between them than with Z. luxurians. This result is congruent with the morphological data.
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