Genomic affinities revealed by GISH suggests intergenomic restructuring between parental genomes of the paleopolyploid genus Zea

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.

Repetitive sequences and structural chromosome alterations promote intraspecific variations in Zea mays L. karyotype

LTR-retrotransposons, knobs and structural chromosome alterations contribute to shape the structure and organization of the Zea mays karyotype. Our initial nuclear DNA content data of Z. mays accessions revealed an intraspecific variation (2 C = 2.00 pg to 2 C = 6.10 pg), suggesting differences in their karyotypes. We aimed to compare the karyotypes of three Z. mays accessions in search of the differences and similarities among them. Karyotype divergences were demonstrated among the accessions, despite their common chromosome number (2n = 20) and ancestral origin. Cytogenomic analyses showed that repetitive sequences and structural chromosome alterations play a significant role in promoting intraspecific nuclear DNA content variation. In addition, heterozygous terminal deletion in chromosome 3 was pointed out as a cause of lower nuclear 2 C value. Besides this, translocation was also observed in the short arm of chromosome 1. Differently, higher 2 C value was associated with the more abundant distribution of LTR-retrotransposons from the family Grande in the karyotype. Moreover, heteromorphism involving the number and position of the 180-bp knob sequence was found among the accessions. Taken together, we provide insights on the pivotal role played by repetitive sequences and structural chromosome alterations in shaping the karyotype of Z. mays.

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, A_mA_mB_mB_m) showed up to 5IV, suggesting pairing between chromosomes from genomes homoeologous A_m and B_m. In untreated individuals of the alloautooctoploid Zea perennis (8x = 2n = 40, A_pA_pA_p´A_p´B_p1B_p1B_p2B_p2) 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 (A_m or B_m) 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.