Molecular Cytogenetic Support for an Evolutionary Model of Zea (Poaceae)

The genus Zea comprises maize and its wild relatives, commonly known as teosintes, whose complex genomic histories reflect ancient whole-genome duplication and subsequent genome restructuring. Molecular cytogenetic analyses provide complementary evidence of genomic affinities and repetitive DNA differentiation among these taxa. Here, we integrate genomic in situ hybridisation (GISH), fluorescent in situ hybridisation (FISH), and DAPI banding to evaluate genomic affinities among maize and representative annual and perennial Zea taxa, and to assess the consistency of the observed patterns with a previously proposed evolutionary model. GISH using total genomic DNA from the teosintes Z. mays subsp. parviglumis and Z. mays subsp. mexicana as probes on mitotic maize chromosomes produced strong and widespread hybridisation signals. In contrast, Z. mays subsp. huehuetenanguensis, Z. luxurians, Z. diploperennis, and Z. perennis showed weaker or more heterogeneous patterns, particularly across chromosome arms and knob-bearing regions. DAPI banding revealed marked differences in the distribution of heterochromatic knobs, including conspicuous terminal bands in Z. luxurians and no detectable DAPI-positive bands in Z. perennis. In Z. mays subsp. parviglumis, FISH with the 180-bp knob repeat produced signals at subtelomeric regions corresponding to DAPI-positive bands. Furthermore, GISH of Z. luxurians chromosomes using labelled Z. luxurians genomic DNA and unlabelled Z. perennis DNA as blocking DNA revealed strong hybridisation signals predominantly at knob regions. Overall, these findings provide complementary molecular cytogenetic evidence of differential sequence conservation and repetitive DNA differentiation among Zea taxa. Considered alongside previous meiotic and genomic evidence, the results are consistent with a model of unequal ancestral subgenome evolution and genome restructuring during polyploidisation and subsequent diploidisation.