Genome size and numerical polymorphism for the B chromosome in races of maize (Zea mays ssp. mays, Poaceae)

Abstract

Twenty-one native populations (1120 individuals) of maize from Northern Argentina were studied. These populations, which belong to 13 native races, were cultivated at different altitudes (80-3620 m). Nineteen of the populations analyzed showed B chromosome (Bs) numerical polymorphism. The frequency of individuals with Bs varied from 0 to 94%. The number of Bs per plant varied from 0 to 8 Bs, with the predominant doses being 0, 1, 2, and 3. Those populations with varying number of Bs showed a positive and statistically significant correlation of mean number of Bs with altitude. The DNA content, in plants without Bs (A-DNA)(2n = 20), of 17 populations of the 21 studied was determined. A 36% variation (5.0-6.8 pg) in A-DNA content was found. A significant negative correlation between A-DNA content and altitude of cultivation and between A-DNA content and mean number of Bs was found. This indicates that there is a close interrelationship between the DNA content of A chromosomes and doses of Bs. These results suggest that there is a maximum limit to the mass of nuclear DNA so that Bs are tolerated as long as this maximum limit is not exceeded.

B Chromosome Polymorphism in Maize Landraces: Adaptivevs. Demographic Hypothesis of Clinal Variation

“…The cytogenetic studies conducted by Rosato et al (1998) and Poggio et al (1998) in 21 native populations from northwestern Argentina revealed a negative correlation between the mean number of B's per plant and the number of heterochromatic bands and a positive correlation between the mean number of B's per plant and altitude. According to these authors, B's would be tolerated at high frequencies in those populations with lower numbers of heterochromatic bands, so as to maintain a relatively constant amount of nuclear DNA or an ''optimal'' nucleotype (sensu Bennett 1987).…”

“…mays L.). Numerical polymorphisms have been described in a large number of maize landraces (Mcclintock et al 1981;Chiavarino et al 1995;Naranjo et al 1995;Rosato et al 1998), with differences in the number of B's being one of the main factors contributing to intraspecific genome-size variation. Although several accumulation mechanisms have been reported for maize B chromosomes (Roman 1947(Roman ,1948Carlson and Chou 1981;Carlson and Roseman 1992), the significance of the distinctive patterns of occurrence of these supernumerary elements in different native populations still remains unclear.…”

“…The rationale behind this approach is that the effects of selection are generally locus specific, whereas demographic processes are expected to have relatively uniform effects across the entire genome (Cavalli-Sforza 1966;Lewontin and Krakauer 1973). Rosato et al (1998) suggested that the altitudinal cline of B chromosomes found in maize landraces from northwestern Argentina could be the product of selective forces. However, no experimental data were provided to support this assertion.…”

“…However, no experimental data were provided to support this assertion. The aim of the present study is to test the adaptive significance of the altitudinal cline of B chromosomes by using neutral molecular markers ½single sequence repeats (SSRs) to assess the levels of genetic divergence of the populations studied by Rosato et al (1998). If the B chromosome gradient is due to demographical processes related to the history of the populations, the levels of divergence estimated by the neutral markers should also exhibit a clinal pattern of variation.…”

Estudios citogenéticos evolutivos del género Zea

“…Las variaciones en el tamaño del genoma se deberían fundamentalmente a variaciones en el contenido de heterocromatina de los cromosomas, sobre todo de aquellas zonas que tienen secuencias repetidas en tándem (Flavel, 1986;Price 1988;Tito et al 1991;Rosato et al, 1998). En cambio para San Miguel et al, 1996Miguel et al, , 1998Okagaki y Phillips, 2004, en genomas complejos como los de los eucariotas, aún no se conoce con exactitud cuál es la organización relativa de los genes y la función del DNA repetido.…”