Karyotype comparation between Zea luxurians and maize Amarillo Chico through DAPI-banding and
FISH of knob sequence
Gonz�lez, GE; Fourasti�, MF; Poggio, L
Departamento de Ecolog�a, Gen�tica y Evoluci�n, Facultad de
Ciencias Exactas y Naturales, Universidad de Buenos Aires,
Argentina. E-mail:
[email protected]
Karyotypes
of Zea luxurians and the maize of race Amarillo Chico from Northwest of Argentina
(NOA) were described using DAPI-banding and Fluorescent In Situ Hybridization (FISH) techniques.
The
variation in DNA content in Zea has been proposed that is largely caused
by differences in the heterochromatin amounts that form distal blocks named
knobs (Laurie and Bennett, Heredity
55:307-313, 1985; Poggio et al., Ann.
Bot. 82:115-117, 1998). These
knobs may vary in quantity and size among the different races of
maize and its wild relatives (Kato,
Agric. Exp. Stn.
Bull. 635: 1-185, 1976; McClintock et al., Col. Postgrad. Chapingo M�xico, 1981). Zea
luxurians possesses the highest
genome size of the 2n=20 Zea
species (2C= 8.83 pg), while the DNA content of maize varies between 2C= 4.92
and 6.79 pg (Tito et al., Theor.
Appl. Genet. 83:58-64, 1991). The heterochromatic knobs from maize and Zea
luxurians correspond to C and DAPI-positive bands on mitotic metaphase
chromosomes (Tito et al., 1991; Poggio et al., 1998), and there
are composed principally by a 180-bp tandem repeat sequence (Peacock et
al., Proc. Natl. Acad.
Sci. USA 78:4490-4494, 1981).
The materials used in this study were the Zea luxurians cv.
9478 (Guatemalan) Leg. CIMMYT and the Argentinean race of maize Amarillo
Chico (VAV 6451) from NOA Leg. by Vavilov Laboratory, University of Buenos
Aires (UBA). The materials were cultivated in the greenhouse of Facultad de
Agronom�a, UBA.
DAPI banding and FISH performed according to Summer (Chromosome banding. Unwin Hyman,
London, 1990) and Gonz�lez et al. (Chrom. Res. 14:629-635, 2006), respectively, were carried out on mitotic metaphases
from Zea luxurians and maize. For
FISH the knob-180bp sequence was obtained and used as probe. Slides were examined with a Carl Zeiss Axiophot
epifluorescence microscope and the photographs were taken using a Leica
CCD digital camera.
Chromosomal parameters were measured in at least 10 metaphases, for each
species, using the freeware program MicroMeasure 3.3 (http:
//www.colostate.edu/depts/biology/micromeasure). For karyotyping the relative
chromosome length, arm ratio y centromeric index were calculated. Chromosomes
were ordered from high to low as usually in maize, and
the chromosomal morphology was described according to Levan et al. (Hereditas 52: 201-220, 1964). Also,
intra and inter-chromosomal asymmetry indexes, A1 and A2 described by Romero
Zarco (Taxon 35: 526-530, 1986),
were calculated. These indexes, in addition
to the formulae and karyotypic parameters from Zea luxurians and the
Amarillo Chico maize are shown in Table 1.
Table 1
|
Maize Amarillo
Chico |
Zea luxurians |
2n=4x |
20 |
20 |
DNA content 2C (X +/- Standart Error)* |
5.63 pg (+/- 0.05) |
8.83 pg (+/- 0.08) |
Karyotypic
Formula |
6 m + 4
sm |
5 m + 4
sm + 1 sm-st |
TCL |
135um |
173um |
TCL without knobs |
124um |
168um |
% of heterochromatin � |
7.65 |
21.16 |
Range of knob sizes |
1.62um - 0.54um |
2.74um - 0.8um |
Media of knob sizes |
1.04um |
1.75um |
A1 |
0.36 |
0.39 |
A2 |
0.27 |
0.15 |
Ref.: * From Tito et
al., 1991. TCL: Total Chromosome
Length. �: Calculated as a percent of TCL. A1:
intra-chromosomal asymmetry index. A2:
inter-chromosomal asymmetry index. m:
metacentric. sm: sub-metacentric. st: sub-telocentric.
The analysis of the karyotype
parameters let us to elaborate the idiograms from the taxa analyzed (Fig. 1). It
is important to note that the knobs 180pb sequence shown FISH-positive signals
on all heterochromatic DAPI-positive bands, as it is indicated in the figure 1.
Figure 1: Idiograms of Zea
luxurians (A) and Maize race Amarillo Chico (B). The white blocks represent
the coincident DAPI-positive bands and the 180bp sequence FISH signals. M: metacentric. Sm: sub-metacentric. St:
sub-telocentric. IC: Centromeric
index.
Tito et al. (1991) and Poggio et al. (1998)
found, applying C and DAPI banding to different maize races from NOA, a
positive relation between the genome size and the heterochromatin percent
corresponding to knobs. We revealed, using DAPI-banding and FISH, that Zea luxurians have about 14% more
knob-heterochromatin than maize, indeed
knobs of Zea luxurians are about 40% bigger than the maize ones (Fig. 1). Moreover, we observed that the TCL of Zea luxurians is 28.6 % higher than
maize, being approximately 26% if knobs are not considered. Also, we have seen
that the Total Chromosome Volume of Zea
luxurians is about 28% higher than maize. Therefore, the differences in DNA content and the chromosome sizes
between both species could be explicated for both its dissimilar number and
size of knobs, and the different amounts of interspersed DNA.
From the observed in
figure 1, we conclude that the difference founded in inter-chromosomal
asymmetry in the taxa studied is due
to the number, size and distribution of heterochromatic knobs on both
chromosomes arms.
Finally, the cytogenetic studies presented here revealed important karyotypic and
chromosomal differences between Zea
luxurians and maize.