Intragenomic Conflict between Knob Heterochromatin and B Chromosomes Is the Key to Understand Genome Size Variation along Altitudinal Clines in Maize

Abstract

In maize, we studied the causes of genome size variation and their correlates with cultivation altitude that suggests the existence of adaptive clines. To discuss the biological role of the genome size variation, we focused on Bolivian maize landraces growing along a broad altitudinal range. These were analyzed together with previously studied populations from altitudinal clines of Northwestern Argentina (NWA). Bolivian populations exhibited numerical polymorphism for B chromosomes (Bs) (from 1 to 5), with frequencies varying from 16.6 to 81.8 and being positively correlated with cultivation altitude. The 2C values of individuals 0B (A-DNA) ranged between 4.73 and 7.71 pg, with 58.33% of variation. The heterochromatic knobs, detected by DAPI staining, were more numerous and larger in individuals 0B than in those with higher doses of Bs. Bolivian and NWA landraces exhibited the same pattern of A-DNA downsizing and fewer and smaller knobs with increasing cultivation altitude, suggesting a mechanistic link among heterochromatin, genome size and phenology. The negative association between the two types of supernumerary DNA (knob heterochromatin and Bs), mainly responsible for the genome size variation, may be considered as an example of intragenomic conflict. It could be postulated that the optimal nucleotype is the result of such conflict, where genome adjustment may lead to an appropriate length of the vegetative cycle for maize landraces growing across altitudinal clines.

Chromosomic Studies in Zephyranthes Citrina Baker (Amaryllidaceae), a Polyploid Ornamental

“…This was further supported by Fourastié et al (2017) for other NWA populations. Recently, González and Poggio (2021) observed that the Bolivian landraces cultivated at higher altitudes have lower DNA content than those growing at lower altitudes (Figure 2). Negative correlations between genome size and altitude of cultivation were also detected in different altitudinal clines from the American continent (Rayburn and Auger, 1990;Díez et al, 2013;Bilinsky et al, 2018).…”

“…Jian et al (2017) also observed a high correlation between genome size and flowering time in inbred lines growing under tropical conditions. These results may explain the negative correlation detected between genome size and percentage of heterochromatin with cultivation altitude (Figure 2) (Tito et al, 1991;Poggio et al, 1998;Fourastié et al, 2017;González and Poggio, 2021). Thus, the fact that maize landraces at high altitudes are precocious and show a reduction in heterochromatin percentage most likely represents an adaptation to a shorter growing season typical of highlands, with natural selection acting on the flowering time across altitudinal clines (Bilinsky et al, 2018).…”

“…retrotransposon families), making up over 70% of the nuclear genome (SanMiguel et al, 1998;Meyers et al, 2001;Realini et al, 2016;Fourastié et al, 2017). The presence of accessory chromosomes (B chromosomes; Bs) also contribute to DNA content variation, as discussed later (Rosato et al, 1998;Fourastié et al, 2017;González and Poggio, 2021).…”

“…In Zea, DNA content is positively correlated with the number and size of knobs, as well as with the percentage of heterochromatin in the karyotype (Laurie and Bennett, 1985;Tito et al, 1991;Poggio et al, 1998;Realini et al, 2016;Fourastié et al, 2017;González and Poggio, 2021). The percentage of heterochromatin in a Zea karyotype is calculated by summing all chromosomal portions occupied by the DAPI-positive bands detected in the chromosomal complement.…”

“…In maize landraces, the percentage of heterochromatin Rosato et al, 1998;Realini et al, 2016;Fourastié et al, 2017;González et al, 2021. is positively correlated with the genome size (González and Poggio, 2011;Realini et al, 2016;Fourastié et al, 2017), suggesting that the variation in DNA content is mainly due to differences in the size, and to a lesser extent, in the number of knobs. However, some authors provided evidence for the presence of other sources than heterochromatin knobs contributing to genome size (revisited in Realini et al, 2016;.…”