Genome size and repetitive sequences are driven by artificial selection on the length of the vegetative cycle in maize landraces from Northeastern Argentina

María Florencia Realini Lidia Poggio Julián Cámara-Hernández Graciela Esther González About the authors

Abstract

Variation in genome size and knob heterochromatin content was explored in relationship to altitudinal cline and length of the vegetative cycle in northern Argentina, USA and Mexico landraces. It was considering that the decrease in DNA and heterochromatin content could be an adaptation to a shorter growing season and the result of artificial selection by man. Guaraní landraces from Northeastern Argentina (NEA) show similar variation in genome size (3.81pg to 7.56pg) and knob heterochromatin content than maize growing across an altitudinal cline. The present analysis offers an overview of the genetic variability of NEA maize to explain why Guaraní landraces and those along an altitudinal cline share this similar variation. Karyotype and flow cytometry data were employed. The DNA content of Guaraní landraces which lacking B chromosomes, showed no significant relationship with knob heterochromatin, suggesting differences in the amount of interspersed DNA. A significant positive relationship was found between the length of the vegetative cycle and both number and percentage of knob heterochromatin. No significant correlation was found between genome size and vegetative cycle. All these results allow us to conclude that the variation in heterochromatin content among Guaraní maize is driven by the selection of farmers for flowering time.

Key words:
genome size; knob heterochromatin; length of vegetative cycle; repetitive sequence variation; selective effect

Resumo

La variación observada en el tamaño del genoma y el contenido de heterocromatina knob en relación con el cline altitudinal y la duración del ciclo vegetativo en razas de maíz nativas del norte de Argentina, Estados Unidos y México, permitió considerar que la disminución del contenido de ADN y heterocromatina se debería a una adaptación a temporadas cortas de crecimiento y al resultado de selección artificial por parte del hombre. Las razas Guaraníes nativas del noreste de Argentina (NEA), cultivadas a bajas altitudes y sin cromosomas B, muestran una variación similar en el tamaño del genoma (3.81 pg a 7.56 pg) y el contenido de heterocromatina knob a aquellos maíces que crecen a lo largo de un cline altitudinal. El presente análisis ofrece una visión general de la variabilidad genética del maíz del NEA y trata de responder por qué estas razas Guaraníes y los maíces que crecen en un cline altitudinal presentan la variación mencionada. Se emplearon datos de cariotipo y citometría de flujo. El contenido de ADN de las razas Guaraníes no mostró correlación significativa con la heterocromatina knob, lo que sugiere diferencias en el contenido de ADN repetitivo disperso. Se encontró una correlación positiva significativa entre la duración del ciclo vegetativo tanto con el número como con el porcentaje de heterocromatina knob. No se encontró una relación significativa entre el tamaño del genoma y el ciclo vegetativo. Estos resultados nos permiten concluir que la variación en el contenido de heterocromatina entre los maíces Guaraníes estaría impulsada por la selección del tiempo de floración realizada por los agricultores.

Palabras claves:
tamaño del genoma; heterocromatina knob; duración del ciclo vegetativo; variabilidad de secuencias repetitivas; efecto selectivo

To the present, 51 morphological maize landraces have been described in northern Argentina (NA): 28 Northwestern (NWA) landraces distributed along an altitudinal cline, and 23 Northeastern (NEA) landraces cultivated without differences in altitude. In NEA, indigenous Guaraní communities from subtropical forests of Misiones Province cultivate up to 15 landraces (Cámara - Hernández et al. 2011Cámara-Hernández J, Miante Alzogaray AM, Bellon R & Galmarini AJ (2011) Razas de maíz nativas de la Argentina. Ed. Facultad de Agronomía, Universidad de Buenos Aires, Buenos Aires. 174p.). In NA landraces, genome size ranges between 4.4 pg and 6.9 pg (Tito et al. 1991Tito C, Poggio L & Naranjo CA (1991) Cytogenetics studies in the genus Zea: DNA content and heterochromatin in species and hybrids. Theoretical and Applied Genetics 83: 58-64.; Poggio et al. 1998Poggio L, Rosato M, Chiavarino M & Naranjo CA (1998) Genome size and environmental correlations in maize. Annals of Botany 82: 115-117.; Realini et al. 2016Realini MF, Poggio L, Cámara-Hernández J & González GE (2016) Intra-specific variation in genome size in maize: cytological and phenotypic correlates. AoB PLANTS 8: plv138. doi:10.1093/aobpla/plv138
https://doi.org/10.1093/aobpla/plv138...
; Fourastié et al. 2017Fourastié MF, Gottlieb AM, Poggio L & González GE (2017) Are cytological parameters of maize landraces (Zea mays ssp. mays) adapted along an altitudinal cline? Journal of Plant Research: 131, 285-296. DOI: <https://doi.org/10.1007/s10265-017-0996-3>
https://doi.org/10.1007/s10265-017-0996-...
; Realini 2017Realini MF (2017) Variabilidad citogenética de los maíces Guaraníes del Noreste de Argentina: caracterización cariotípica, tamaño del genoma y correlatos fenotípicos. Doctoral’s Thesis. Universidad de Buenos Aires-UBA, Buenos Aires. 194p.). A similar genome size variation was reported for maize growing along altitudinal clines in USA and Mexico (Laurie & Bennett 1985Laurie DA & Bennett MD (1985) Nuclear DNA content in the genera Zea and Sorghum. Intergeneric, interespecific and intraspecific variation. Heredity 55: 307-313.; Rayburn et al. 1985Rayburn AL, Price HJ, Smith JD & Gold JR (1985) C-Band Heterochromatin and DNA Content in Zea mays. American Journal of Botany 72: 1610-1617.; Rayburn & Auger 1990aRayburn AL & Auger JA (1990a) Nuclear DNA content variation in the ancient indigenous races of Mexican maize. Acta Botanica Neerlandica 39: 197-202., bRayburn AL & Auger JA (1990b) Genome size variation in Zea mays ssp. Mays adapted to different altitudes. Theoretical and Applied Genetics 79: 470-474.; Díez et al. 2013Díez CM, Gaut BS, Meca E, Scheinvar E, Montes-Hernandez S, Eguiarte LE & Tenaillon MI (2013) Genome size variation in wild and cultivated maize along altitudinal gradients. New Physiology 1: 264-276.; Bilinsky et al. 2017)

Genome size variation in maize has been mainly attributed to differences in the amount of heterochromatin, principally due to the presence of heterochromatin knobs and of B chromosomes (Bs) (Laurie & Bennett 1985Laurie DA & Bennett MD (1985) Nuclear DNA content in the genera Zea and Sorghum. Intergeneric, interespecific and intraspecific variation. Heredity 55: 307-313.; Poggio et al. 1998Poggio L, Rosato M, Chiavarino M & Naranjo CA (1998) Genome size and environmental correlations in maize. Annals of Botany 82: 115-117.; Fourastié et al. 2017Fourastié MF, Gottlieb AM, Poggio L & González GE (2017) Are cytological parameters of maize landraces (Zea mays ssp. mays) adapted along an altitudinal cline? Journal of Plant Research: 131, 285-296. DOI: <https://doi.org/10.1007/s10265-017-0996-3>
https://doi.org/10.1007/s10265-017-0996-...
). Other proposed cause includes differences in the amount of interspersed DNA (SanMiguel & Bennetzen 1998SanMiguel P & Bennetzen JL (1998) Evidence that a recent increase in maize genome size was caused by the massive amplification of intergene retrotransposons. Annals of Botany 82: 37-44.; Meyers et al. 2001Meyers BC, Tingey SV & Morgante M (2001) Abundance, distribution, and transcriptional activity of repetitive elements in the maize genome. Genome Research 11: 1660-1676.; Bilinski et al. 2018Bilinski P, Albert PS, Berg JJ, Birchler JA, Grote MN, Lorant A et al. (2018) Parallel altitudinal clines reveal trends in adaptive evolution of genome size in Zea mays. PLoS Genetics 14: e1007162. <https://doi.org/10.1371/journal.pgen.1007162>
https://doi.org/10.1371/journal.pgen.100...
). Although Bs are common in NWA landraces, showing large intra- and inter-population differences in number and frequency (Fourastié et al. 2017Fourastié MF, Gottlieb AM, Poggio L & González GE (2017) Are cytological parameters of maize landraces (Zea mays ssp. mays) adapted along an altitudinal cline? Journal of Plant Research: 131, 285-296. DOI: <https://doi.org/10.1007/s10265-017-0996-3>
https://doi.org/10.1007/s10265-017-0996-...
; Poggio et al. 1998Poggio L, Rosato M, Chiavarino M & Naranjo CA (1998) Genome size and environmental correlations in maize. Annals of Botany 82: 115-117.; Rosato et al. 1998Rosato M, Chiavarino A, Naranjo C, Cámara - Hernández J & Poggio L (1998) Genome size and numerical polymorphism for B- chromosome races of maize (Zea mays ssp. mays, Poaceae). American Journal of Botany 85: 168-174.), they have not been previously detected in Guaraní landraces from NEA (Realini et al. 2018Realini MF, Poggio L, Cámara-Hernández J & González GE (2018) Exploring karyotype diversity of Argentinian Guaraní maize landraces: relationship among South American maize. PLOS ONE 13: e0198398.). In Argentinian landraces, variation in genome size and knob heterochromatin content was explored in relationship to altitudinal cline and length of the vegetative cycle (Fourastié et al. 2017Fourastié MF, Gottlieb AM, Poggio L & González GE (2017) Are cytological parameters of maize landraces (Zea mays ssp. mays) adapted along an altitudinal cline? Journal of Plant Research: 131, 285-296. DOI: <https://doi.org/10.1007/s10265-017-0996-3>
https://doi.org/10.1007/s10265-017-0996-...
; Realini et al. 2016Realini MF, Poggio L, Cámara-Hernández J & González GE (2016) Intra-specific variation in genome size in maize: cytological and phenotypic correlates. AoB PLANTS 8: plv138. doi:10.1093/aobpla/plv138
https://doi.org/10.1093/aobpla/plv138...
; Realini 2017Realini MF (2017) Variabilidad citogenética de los maíces Guaraníes del Noreste de Argentina: caracterización cariotípica, tamaño del genoma y correlatos fenotípicos. Doctoral’s Thesis. Universidad de Buenos Aires-UBA, Buenos Aires. 194p.). The decrease in knob heterochromatin found at high altitudes was proposed to be related to the length of the growing season (Poggio et al. 1998Poggio L, Rosato M, Chiavarino M & Naranjo CA (1998) Genome size and environmental correlations in maize. Annals of Botany 82: 115-117.) and to natural selection on flowering time across altitudinal clines (Bilinsky et al. 2017). Then, the following question arises: why do landraces living in sympatry in lowland restricted areas and those along an altitudinal cline share similar variation in genome size and heterochromatin content?

In this study, we investigated the parallel link of the vegetative cycle length with variation in intraspecific genome size and abundance of knob heterochromatin in Guaraní landraces from Northeastern Argentina (NEA). New genome size and heterochromatin data were estimates for NEA maize populations and were integrated with those previously reported for Guaraní maize populations (Realini et al. 2016Realini MF, Poggio L, Cámara-Hernández J & González GE (2016) Intra-specific variation in genome size in maize: cytological and phenotypic correlates. AoB PLANTS 8: plv138. doi:10.1093/aobpla/plv138
https://doi.org/10.1093/aobpla/plv138...
, 2018Realini MF, Poggio L, Cámara-Hernández J & González GE (2018) Exploring karyotype diversity of Argentinian Guaraní maize landraces: relationship among South American maize. PLOS ONE 13: e0198398.; Realini 2017Realini MF (2017) Variabilidad citogenética de los maíces Guaraníes del Noreste de Argentina: caracterización cariotípica, tamaño del genoma y correlatos fenotípicos. Doctoral’s Thesis. Universidad de Buenos Aires-UBA, Buenos Aires. 194p.). The present jointly analysis offer a more comprehensive overview of the genetic variability source of the native maize of NEA region and allows us to develop a general discussion about the selective forces involved (Tab. 1).

Table 1
Percentages of heterochromatin, number of knobs, genome size and length of the vegetative cycle.

The maize samples analyzed here were collected from Guaraní farms in Misiones Province, Argentina: VAV6843- Pororó Chico from Aldea Perutí, El Alcázar Depto. Libertador General San Martín; VAV6823- Overo, from Aldea Yuytu Pará, Ruíz de Montoya; VAV6604- Colorado from Pozo Azul, Depto. San Pedro; VAV2011/09- Pipoca Amarillo from Aldea Pozo Azul, Depto. Eldorado. The specimens were deposited at the seed bank of the Plant Genetic Resources Laboratory “N. I. Vavilov” in the Facultad de Agronomía, Universidad de Buenos Aires. DNA content was measured in three to five individuals from each ears and two to five ears per population, with three replicates per individual and data analysis was carried out according to Realini et al. (2016Realini MF, Poggio L, Cámara-Hernández J & González GE (2016) Intra-specific variation in genome size in maize: cytological and phenotypic correlates. AoB PLANTS 8: plv138. doi:10.1093/aobpla/plv138
https://doi.org/10.1093/aobpla/plv138...
). The 2C DNA content was measured in three to five individuals of each corncob and two to five corncobs per population, with thee replicates per individual. The cell nucleus were stained with propidium iodide (PI). Pisum sativum cv. citrad (9.09 pg), used as internal standard, was kindly provided by Dr J. Doležel from the Institute of Experimental Botany, Sokolovska, Czech Republic (Doležel et al. 2007Doležel J, Greilhuber J & Suda J (2007) Estimation of nuclear DNA content in plants using flow cytometry. Nature Protocols 2: 2233-2244.). For each individual, 100 mg offresh leave samples were co-chopped with 50 mg of P. sativum leaves in a Petri dish with 0.5 mL of buffer Otto I (citric acid 0.1 M and 0.5 % v/v of Tween 20), usinga stainless-steel razor blade. The sample was filtered through a nylon mesh (45 µm pore size) and then 0.5 mL of buffer Otto II (0.4 M Na2HPO4 12H2O) supplementedwith PI (50 µg mL-1 of final concentration) and RNase (50 µg mL-1 of final concentration) were added. The samples were incubated in the dark for 40 min. Flow cytometry was performed at Instituto Nacional de Tecnología Agropecuaria (INTA-Catelar) with a CyFlow Ploidy Analyzer Cytometer (Partec). We adjusted the gain to 400 and the sample speed to 0.4 mL-1. The samples were run until 5,000 nuclei were scored. The DNA content was estimated from gated fluorescence histograms of the PI area. Data analysis was performed using the software Flowing 2.5.0 (<http://www.flowingsoftware.com>). Genome size (2C DNA, in picograms) was determined comparing the peak of the sample to the peak of the standard according to Doležel et al. (2007) Doležel J, Greilhuber J & Suda J (2007) Estimation of nuclear DNA content in plants using flow cytometry. Nature Protocols 2: 2233-2244.. All samples with a coefficient of variance ≤ 5 were included in the present study. Conversion from picograms to megabase pairs was done according to Doležel et al. (2003)Doležel J, Bartŏs J, Voglmayr H & Greilhuber J (2003) Nuclear DNA content and genome size of trout and human. Cytometry A 51: 127-128..

All statistical analyses were performed using the programs Infostat, FCA, National University of Córdoba (Di Rienzo et al. 2012Di Rienzo JA, Casanoves F, Balzarini MG, González L, Tablada M & Robledo CW (2012) InfoStat. version 2012. FCA-UNC, Grupo InfoStat, Córdoba, Available: <http://www.infostat.com.ar/>.
http://www.infostat.com.ar/...
) and R (R Development Core Team 2012R Development Core Team (2012) R: a language and environment for statistical computing. R Foundation for Statistical Computing, Vienna. Available at <https://www.r-project.org/>.
https://www.r-project.org/...
). Statistical significance was set at P < 0.05.

The variation in genome size of the Argentinian Guaraní maize populations was analyzed using analysis of variance (ANOVA), followed by the Tukey’s HSD test for pairwise comparisons. The DNA content (2C values) varied from 3.81pg to 7.56pg, and there were significant differences among populations (Pv ˂ 0.0001, F 23.501 = 7.25, n = 578, Tab. 1). In addition, intra-population variation in DNA content was evaluated through the intraclass correlation coefficient at the ear level (ICCear = 0.252) and at the individual-within-ear level (ICCindividual/ear = 0.818). The individual and ear random effects accounted for about 82% of the total residual variance, while the ear random effect alone explained only about 25% of the variance. The inter-population variation was similar to those obtained in previous studies involving lines and landraces from Argentina, Mesoamerica and USA (Laurie & Bennett 1985Laurie DA & Bennett MD (1985) Nuclear DNA content in the genera Zea and Sorghum. Intergeneric, interespecific and intraspecific variation. Heredity 55: 307-313.; Rayburn & Auger 1990aRayburn AL & Auger JA (1990a) Nuclear DNA content variation in the ancient indigenous races of Mexican maize. Acta Botanica Neerlandica 39: 197-202., bRayburn AL & Auger JA (1990b) Genome size variation in Zea mays ssp. Mays adapted to different altitudes. Theoretical and Applied Genetics 79: 470-474.; Tito et al. 1991Tito C, Poggio L & Naranjo CA (1991) Cytogenetics studies in the genus Zea: DNA content and heterochromatin in species and hybrids. Theoretical and Applied Genetics 83: 58-64.; Poggio et al. 1998Poggio L, Rosato M, Chiavarino M & Naranjo CA (1998) Genome size and environmental correlations in maize. Annals of Botany 82: 115-117.; Díez et al. 2013Díez CM, Gaut BS, Meca E, Scheinvar E, Montes-Hernandez S, Eguiarte LE & Tenaillon MI (2013) Genome size variation in wild and cultivated maize along altitudinal gradients. New Physiology 1: 264-276.; Bilinsky et al 2017; Fourastié et al. 2017Fourastié MF, Gottlieb AM, Poggio L & González GE (2017) Are cytological parameters of maize landraces (Zea mays ssp. mays) adapted along an altitudinal cline? Journal of Plant Research: 131, 285-296. DOI: <https://doi.org/10.1007/s10265-017-0996-3>
https://doi.org/10.1007/s10265-017-0996-...
).

In regard to knobs repetitive DNA, the Guaraní maize populations showed variation in number (from 8 to 23), and percentage of knob heterochromatin (from 5.30 to 20.02% of total chromosome length) (Fig. 1) (Realini 2017Realini MF (2017) Variabilidad citogenética de los maíces Guaraníes del Noreste de Argentina: caracterización cariotípica, tamaño del genoma y correlatos fenotípicos. Doctoral’s Thesis. Universidad de Buenos Aires-UBA, Buenos Aires. 194p.; Realini et al. 2018Realini MF, Poggio L, Cámara-Hernández J & González GE (2018) Exploring karyotype diversity of Argentinian Guaraní maize landraces: relationship among South American maize. PLOS ONE 13: e0198398.). Moreover, variation in the number of knobs and percentage of heterochromatin has been reported at the intra-population level (Tab. 1) (Realini et al. 2016Realini MF, Poggio L, Cámara-Hernández J & González GE (2016) Intra-specific variation in genome size in maize: cytological and phenotypic correlates. AoB PLANTS 8: plv138. doi:10.1093/aobpla/plv138
https://doi.org/10.1093/aobpla/plv138...
, 2018Realini MF, Poggio L, Cámara-Hernández J & González GE (2018) Exploring karyotype diversity of Argentinian Guaraní maize landraces: relationship among South American maize. PLOS ONE 13: e0198398.; Realini 2017Realini MF (2017) Variabilidad citogenética de los maíces Guaraníes del Noreste de Argentina: caracterización cariotípica, tamaño del genoma y correlatos fenotípicos. Doctoral’s Thesis. Universidad de Buenos Aires-UBA, Buenos Aires. 194p.). Analysis of karyotype data showed that the correlation between the number of knobs and percentage of knob heterochromatin (Tab. 1), using the Spearman coefficient (SC), was positively significant (Pv ˂ 0.0001, SC = 0.85; n = 105; Y = -2.05 + 0.86 X, R = 0.72, Pv ˂ 0.0001). It is important to point out that individuals of the same population with the same number of knobs, not always exhibited similar percentages of heterochromatin. This suggests that the percentage of heterochromatin depend the number of knob and the numbers of copies of the satellite DNA repeats that conform them (Fig. 1). Although, thus allowed us to suggest that the variation in the number of copies of repetitive scattered DNA sequences are important sources of content variation (SanMiguel & Bennetzen 1998SanMiguel P & Bennetzen JL (1998) Evidence that a recent increase in maize genome size was caused by the massive amplification of intergene retrotransposons. Annals of Botany 82: 37-44.; Meyers et al. 2001Meyers BC, Tingey SV & Morgante M (2001) Abundance, distribution, and transcriptional activity of repetitive elements in the maize genome. Genome Research 11: 1660-1676.; Tenaillon et al. 2011Tenaillon MI, Hufford MB, Gaut BS & Ross-Ibarra J (2011) Genome size and transposable element content as determined by high-throughput sequencing in Maize and Zea luxurians. Genome Biology and Evolution 3: 219-229., 2016Tenaillon MI, Manicacci D, Nicolas SD, Tardieu F & Welcker C (2016) Testing the link between genome size and growth rate in maize. PeerJ 4:e2408 DOI: <https://doi.org/10.7717/peerj.2408PeerJ4:e2408>
https://doi.org/10.7717/peerj.2408PeerJ4...
; Chia et al. 2012; Bilinsky et al. 2017).

Figure 1
a-f. Comparative analysis of knob chromosomal distribution, estimated by FISH - a. Cariograms DAPI/ FISH in six Guaraní maize landraces (1 = V6563, Tupí Amarillo; 2 = VAV6607, Pipoca Colorado; 3 = VAV6560, Blanco Ancho; 4 = VAV6575, Pororó Chico; 5 = VAV6565, Rosado; 6 = VAV6574, Blanco Angosto). b-f. Metaphase chromosomes by FISH with knobs, 18S and 5S rDNA probes - b. VAV6592, Tupí Blanco; c. VAV6562, Pororó Grande; d. VAV6565, Rosado; e. VAV6556, Amarillo Chico; f. VAV6837, Colorado. The probes were labeled with digoxigenin and biotin, and revealed with antidigoxigenin-FITC (green) and Cy3 (red), respectively. Ref. The violet arrowhead indicated the 6L2 and 6L3 knob positions. The white arrowhead indicates knobs hybridized only with single knob sequence. The numbers indicate the chromosomal pairs. Scale bars = 10µm.

The DNA content of the Guaraní populations showed no relationship either with the number of knobs (Pv = 0.2106, SC = 0.30, df = 18) or with the percentage of heterochromatin (Pv = 0.1110, SC = 0.38, df = 18). In Figure 2 are represented the relations between the abundance of repeat DNA from knob heterochromatin, genome size and length of the vegetative cycle in Guaraní landraces, interestingly not always the populations with a major percentage of heterochromatin content has the highest values of genome size. In contrast, a recent study carried out on NWA maize showed that the percentage of knob heterochromatin in a set of A chromosomes (A-HC) and DNA amount were positively correlated (Fourastié et al. 2017Fourastié MF, Gottlieb AM, Poggio L & González GE (2017) Are cytological parameters of maize landraces (Zea mays ssp. mays) adapted along an altitudinal cline? Journal of Plant Research: 131, 285-296. DOI: <https://doi.org/10.1007/s10265-017-0996-3>
https://doi.org/10.1007/s10265-017-0996-...
). Thus suggesting that the balance between knob heterochromatin and genome size is different in Guaraní maize than NWA landraces, this could explain for the presence of B chromosomes in NWA landraces. In the present work, the joint analysis of published and new data reveals positive significant relationships between the length of the vegetative cycle and both heterochromatin percentage (Pv = 0.0068, SC = 0.69, df = 13) and knob number (Pv = 0.0007, SC = 0.80, df = 13) (Fig. 2). A clear relationship between genome size and vegetative cycle length was not found in Guaraní landraces (P = 0.66779, SC = 0.13, df = 13) (Fig. 2). In fact, the populations VAV6563 and VAV6568, have 6.26pg and 4.70pg as average values of genome size and they possess longer vegetative cycle, 82 and 84 days, respectively. Despite the differences in DNA content, both populations have a higher percentage of heterochromatin (16.71% and 12.24%, respectively, Fig. 2).

Figure 2
Correlations between abundance of repeat DNA from knob heterochromatin, genome size and length of vegetative cycle in Guaraní landraces. Ref. Solid filled bars show the average percentages of the heterochromatin knob. Bars with crossed lines indicate the means values of genome size. Whiskers represent standard errors.

The data of the present work support that vegetative cycle is more related with heterochromatin than with total DNA content. This could be explained taken into account that the knob heterochromatin is the last component in completing DNA replication since increased DNA packaging leads to a longer synthesis, resulting in a longer cell cycle that may impact on the rate of cell division and plant development (Pryor et al. 1980Pryor A, Faulkner K, Rhoades MM & Peacock WJ (1980) Asynchronous replication of heterochromatin in maize. Proceedings of the National Academy of Sciences of the USA 77: 6705-6709.; Buckler et al. 1999Buckler E, Phelps-Durr TL, Buckler CS, Dawe RK, Doebley JF & Holtsford TP (1999) Meiotic drive of chromosomal knobs reshaped the maize genome. Genetics 153: 415-426.; Greilhuber & Leitch 2013Greilhuber J & Leitch IJ (2013) Genome size and the Phenotype. In: Leitch IJ, Greilhuber J, Dolezel J & Wendel JF (eds.) Plant genome diversity. Physical structure, behaviour and evolution of plant genomes. Vol. 2. Springer, Vienna. Pp. 323-344.). In fact, several studies on maize growing at high altitudes have determined that large amounts of heterochromatin are not favored under harsher climates and shorter growing seasons (Reeves et al. 1998Reeves G, Francis D, Davies MS, Rogers HJ & Hodkinson TR (1998) Genome size is negatively correlated with altitude in natural populations of Dactylis glomerata. Annal of Botany 82: 99-105.; Buckler et al. 1999Buckler E, Phelps-Durr TL, Buckler CS, Dawe RK, Doebley JF & Holtsford TP (1999) Meiotic drive of chromosomal knobs reshaped the maize genome. Genetics 153: 415-426.; Poggio et al. 1998Poggio L, Rosato M, Chiavarino M & Naranjo CA (1998) Genome size and environmental correlations in maize. Annals of Botany 82: 115-117.; Rayburn et al. 1994Rayburn AL, Dudley JW & Biradar DP (1994) Selection for early flowering results in simultaneous selection for reduced nuclear-DNA content in maize. Plant Breeding 112: 318-322.; Bilinsky et al. 2017; Fourastié et al. 2017Fourastié MF, Gottlieb AM, Poggio L & González GE (2017) Are cytological parameters of maize landraces (Zea mays ssp. mays) adapted along an altitudinal cline? Journal of Plant Research: 131, 285-296. DOI: <https://doi.org/10.1007/s10265-017-0996-3>
https://doi.org/10.1007/s10265-017-0996-...
).

The analysis of the data obtained led us to propose that the populations of NEA analyzed here, which grew in similar altitudinal, climatic and / or ecological conditions, are isolated by temporary pre-zigotic barriers through differences in the duration of the vegetative cycle (ie, the flowering time). This isolation is developed artificially by the farmers with the purpose of maintaining the morphological characteristics of each race, avoiding the presence of hybrids with undesirable intermediate morphological and agronomic characteristics. The selection in the flowering time implies an indirect artificial selection for differences in heterochromatin content.

We are grateful to the Guaraní communities in Argentina for providing the maize landraces used in this study to Lic. L. Babino for statistical analysis assistance and to Silvia Pietrokovsky, who reviewed the English of the manuscript.

Acknowledgments

This work was supported by Agencia Nacional de Promoción Científica y Técnica, PICT-2015-2292.

References

  • Bilinski P, Albert PS, Berg JJ, Birchler JA, Grote MN, Lorant A et al (2018) Parallel altitudinal clines reveal trends in adaptive evolution of genome size in Zea mays. PLoS Genetics 14: e1007162. <https://doi.org/10.1371/journal.pgen.1007162>
    » https://doi.org/10.1371/journal.pgen.1007162
  • Buckler E, Phelps-Durr TL, Buckler CS, Dawe RK, Doebley JF & Holtsford TP (1999) Meiotic drive of chromosomal knobs reshaped the maize genome. Genetics 153: 415-426.
  • Cámara-Hernández J, Miante Alzogaray AM, Bellon R & Galmarini AJ (2011) Razas de maíz nativas de la Argentina. Ed. Facultad de Agronomía, Universidad de Buenos Aires, Buenos Aires. 174p.
  • Di Rienzo JA, Casanoves F, Balzarini MG, González L, Tablada M & Robledo CW (2012) InfoStat. version 2012. FCA-UNC, Grupo InfoStat, Córdoba, Available: <http://www.infostat.com.ar/>.
    » http://www.infostat.com.ar/
  • Díez CM, Gaut BS, Meca E, Scheinvar E, Montes-Hernandez S, Eguiarte LE & Tenaillon MI (2013) Genome size variation in wild and cultivated maize along altitudinal gradients. New Physiology 1: 264-276.
  • Doležel J, Bartŏs J, Voglmayr H & Greilhuber J (2003) Nuclear DNA content and genome size of trout and human. Cytometry A 51: 127-128.
  • Doležel J, Greilhuber J & Suda J (2007) Estimation of nuclear DNA content in plants using flow cytometry. Nature Protocols 2: 2233-2244.
  • Fourastié MF, Gottlieb AM, Poggio L & González GE (2017) Are cytological parameters of maize landraces (Zea mays ssp. mays) adapted along an altitudinal cline? Journal of Plant Research: 131, 285-296. DOI: <https://doi.org/10.1007/s10265-017-0996-3>
    » https://doi.org/10.1007/s10265-017-0996-3
  • Greilhuber J & Leitch IJ (2013) Genome size and the Phenotype. In: Leitch IJ, Greilhuber J, Dolezel J & Wendel JF (eds.) Plant genome diversity. Physical structure, behaviour and evolution of plant genomes. Vol. 2. Springer, Vienna. Pp. 323-344.
  • Laurie DA & Bennett MD (1985) Nuclear DNA content in the genera Zea and Sorghum. Intergeneric, interespecific and intraspecific variation. Heredity 55: 307-313.
  • Meyers BC, Tingey SV & Morgante M (2001) Abundance, distribution, and transcriptional activity of repetitive elements in the maize genome. Genome Research 11: 1660-1676.
  • Poggio L, Rosato M, Chiavarino M & Naranjo CA (1998) Genome size and environmental correlations in maize. Annals of Botany 82: 115-117.
  • Pryor A, Faulkner K, Rhoades MM & Peacock WJ (1980) Asynchronous replication of heterochromatin in maize. Proceedings of the National Academy of Sciences of the USA 77: 6705-6709.
  • R Development Core Team (2012) R: a language and environment for statistical computing. R Foundation for Statistical Computing, Vienna. Available at <https://www.r-project.org/>.
    » https://www.r-project.org/
  • Rayburn AL, Price HJ, Smith JD & Gold JR (1985) C-Band Heterochromatin and DNA Content in Zea mays. American Journal of Botany 72: 1610-1617.
  • Rayburn AL & Auger JA (1990a) Nuclear DNA content variation in the ancient indigenous races of Mexican maize. Acta Botanica Neerlandica 39: 197-202.
  • Rayburn AL & Auger JA (1990b) Genome size variation in Zea mays ssp. Mays adapted to different altitudes. Theoretical and Applied Genetics 79: 470-474.
  • Rayburn AL, Dudley JW & Biradar DP (1994) Selection for early flowering results in simultaneous selection for reduced nuclear-DNA content in maize. Plant Breeding 112: 318-322.
  • Realini MF, Poggio L, Cámara-Hernández J & González GE (2016) Intra-specific variation in genome size in maize: cytological and phenotypic correlates. AoB PLANTS 8: plv138. doi:10.1093/aobpla/plv138
    » https://doi.org/10.1093/aobpla/plv138
  • Realini MF (2017) Variabilidad citogenética de los maíces Guaraníes del Noreste de Argentina: caracterización cariotípica, tamaño del genoma y correlatos fenotípicos. Doctoral’s Thesis. Universidad de Buenos Aires-UBA, Buenos Aires. 194p.
  • Realini MF, Poggio L, Cámara-Hernández J & González GE (2018) Exploring karyotype diversity of Argentinian Guaraní maize landraces: relationship among South American maize. PLOS ONE 13: e0198398.
  • Reeves G, Francis D, Davies MS, Rogers HJ & Hodkinson TR (1998) Genome size is negatively correlated with altitude in natural populations of Dactylis glomerata. Annal of Botany 82: 99-105.
  • Rosato M, Chiavarino A, Naranjo C, Cámara - Hernández J & Poggio L (1998) Genome size and numerical polymorphism for B- chromosome races of maize (Zea mays ssp. mays, Poaceae). American Journal of Botany 85: 168-174.
  • SanMiguel P & Bennetzen JL (1998) Evidence that a recent increase in maize genome size was caused by the massive amplification of intergene retrotransposons. Annals of Botany 82: 37-44.
  • Tenaillon MI, Hufford MB, Gaut BS & Ross-Ibarra J (2011) Genome size and transposable element content as determined by high-throughput sequencing in Maize and Zea luxurians. Genome Biology and Evolution 3: 219-229.
  • Tenaillon MI, Manicacci D, Nicolas SD, Tardieu F & Welcker C (2016) Testing the link between genome size and growth rate in maize. PeerJ 4:e2408 DOI: <https://doi.org/10.7717/peerj.2408PeerJ4:e2408>
    » https://doi.org/10.7717/peerj.2408PeerJ4:e2408
  • Tito C, Poggio L & Naranjo CA (1991) Cytogenetics studies in the genus Zea: DNA content and heterochromatin in species and hybrids. Theoretical and Applied Genetics 83: 58-64.

Publication Dates

  • Publication in this collection
    18 Jan 2021
  • Date of issue
    2021

History

  • Received
    29 Dec 2018
  • Accepted
    30 Oct 2019
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