Polyploid speciation in Zea (Poaceae): cytogenetic insights.

MAIN CONCLUSION: The analysis of meiotic pairing affinities and genomic formulae in species and hybrids of Zea allowed us to speculate an evolutionary model to recreate the ancient polyploidization of maize and allied species. The meiotic pairing affinities and the genomic formulae analysis in Zea species and hybrids obtained in new and previous crosses, together with the molecular data known in the genus, allowed us to speculate an evolutionary model to attempt to recreate the ancient polyploidization process of Zea species. We propose that x = 5 semispecies are the ancestors of all modern species of the genus. The complex evolutionary process that originated the different taxa could be included hybridization between sympatric diploid ancestral semispecies (2n = 10) and recurrent duplication of the hybrid chromosome number, resulting in distinct auto- and allopolyploids. After the merger and doubling of independent genomes would have undergone cytological and genetical diploidization, implying revolutionary changes in genome organization and genic balance processes. Based on the meiotic behaviour of the 2n = 30 hybrids, that showed homoeology between the A subgenomes of all parental species, we propose that this subgenome A would be pivotal in all the species and would have conserved the rDNA sequences and the pairing regulator locus (PrZ). In the hypothetical model postulated here, the ancestral semispecies with the pivotal subgenome A would have had a wide geographic distribution, co-occurring and hybridizing with the semispecies harbouring B subgenomes, thus enabling sympatric speciation.

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

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.

Exploring karyotype diversity of Argentinian Guaraní maize landraces: Relationship among South American maize.

In Argentina there are two different centers of maize diversity, the Northeastern (NEA) and the Northwestern (NWA) regions of the country. In NEA, morphological studies identified 15 landraces cultivated by the Guaraní communities in Misiones Province. In the present study we analyzed the karyotype diversity of 20 populations of Guaraní maize landraces through classical and molecular cytogenetic analyses. Our results demonstrate significant intra and inter-populational variation in the percentage, number, size, chromosome position and frequencies of the heterochromatic blocks, which are called knobs. Knob sequence analysis (180-bp and TR-1) did not show significant differences among Guaraní populations. B chromosomes were not detected, and abnormal 10 (AB10) chromosomes were found with low frequency (0.1≥f ≤0.40) in six populations. Our results allowed karyotypic characterization of each analyzed population, defining for the first time the chromosomal constitution of maize germplasm from NEA. The multivariate analysis (PCoA and UPGMA) of karyotype parameters allowed the distinction between two populations groups: the Popcorn and the Floury maize populations. These results are in agreement with previously published microsatellite and morphological/phenological studies. Finally, we compared our karyotype results with those previously reported for NWA and Central Region of South America maize. Our data suggest that there are important differences between maize from NEA and NWA at the karyotype level, supporting the hypothesis that there are two pathways of input of South America maize. Our results also confirm the existence of two centers of diversification of Argentinian native maize, NWA and NEA. This work contributes new knowledge about maize diversity, which is relevant for future plans to improve commercial maize, and for conservation of agrobiodiversity.

Cytological diploidization of paleopolyploid genus Zea: Divergence between homoeologous chromosomes or activity of pairing regulator genes?

Cytological diploidization process is different in autopolyploid and allopolyploid species. Colchicine applied at the onset of meiosis suppresses the effect of pairing regulator genes resulting multivalents formation in bivalent-forming species. Colchicine treated maizes (4x = 2n = 20, AmAmBmBm) showed up to 5IV, suggesting pairing between chromosomes from genomes homoeologous Am and Bm. In untreated individuals of the alloautooctoploid Zea perennis (8x = 2n = 40, ApApAp´Ap´Bp1Bp1Bp2Bp2) the most frequent configuration was 5IV+10II (formed by A and B genomes, respectively). The colchicine treated Z. perennis show up to 10IV revealing higher affinity within genomes A and B, but any homology among them. These results suggest the presence of a paring regulator locus (PrZ) in maize and Z. perennis, whose expression is suppressed by colchicine. It could be postulated that in Z. perennis, PrZ would affect independently the genomes A and B, being relevant the threshold of homology, the fidelity of pairing in each genomes and the ploidy level. Cytological analysis of the treated hexaploid hybrids (6x = 2n = 30), with Z. perennis as a parental, strongly suggests that PrZ is less effective in only one doses. This conclusion was reinforced by the homoeologous pairing observed in untreated dihaploid maizes, which showed up to 5II. Meiotic behaviour of individuals treated with different doses of colchicine allowed to postulate that PrZ affect the homoeologous association by controlling entire genomes (Am or Bm) rather than individual chromosomes. Based on cytological and statistical results it is possible to propose that the cytological diploidization in Zea species occurs by restriction of pairing between homoeologous chromosomes or by genetical divergence of the homoeologous chromosomes, as was observed in untreated Z. mays ssp. parviglumis. These are independent but complementary systems and could be acting jointly in the same nucleus.

Are cytological parameters of maize landraces (Zea mays ssp. mays) adapted along an altitudinal cline?

The Northwestern Argentina (NWA) highland region is one of the southernmost areas of native maize cultivation. We studied variations of different cytological parameters, such as DNA contents, presence/absence of B chromosomes (Bs), and number and sequence composition of heterochromatic knobs in ten accessions of four maize landraces growing along a broad altitudinal cline in NWA. The aim of this work was to assess variations in cytological parameters and their relationship with the crop altitude of cultivation, in an adaptive context. The A-DNA content of the A chromosome complements showed 40% of difference between the lowest (4.5 pg) and the highest (6.3 pg) 2C value. This variation could be attributed to differences in number and size of heterochromatic knobs. Fluorescent in situ hybridization studies revealed the sequence composition of each knob, with a higher proportion of knobs composed of 180-bp repeats rather than TR-1 repeats, in all accessions. We also found numerical polymorphisms and the highest frequency of Bs reported in maize to this date. These results lead us to propose that the frequencies and doses of Bs are influenced by the landrace genotypical make-up. The Bs might be maintained in higher frequencies in those accessions having lower heterochromatin content, so as to preserve an optimal nucleotype. Furthermore, selective forces acting along the altitudinal gradient might be modulating the cytological parameters studied, as suggested by the significant correlations found among them.