Chromosome morphometry and nuclear DNA content are useful data for cytotaxonomy and to understand the evolutionary history of different taxa. For the genus Bromus Linnaeus, 1753, distinct ploidy levels have been reported, occurring from diploid to duodecaploid species. The geographic distribution of Bromus species has been correlated with chromosome number and ploidy level. In this study, the aims were to determine the nuclear genome size and characterize the karyotype of the South American Bromus species: Bromus auleticus Trinius ex Nees, 1829, Bromus brachyanthera Döll, 1878 and Bromus catharticus Vahl, 1791. The mean nuclear 2C value ranged from 2C = 12.64 pg for B. catharticus to 2C = 17.92 pg for B. auleticus, meaning a maximum variation of 2C = 5.28 pg, equivalent to 41.70%. Despite this significant difference in 2C value, the three species exhibit the same chromosome number, 2n = 6x = 42, which confirms their hexaploid origin. Corroborating the genome size, the chromosome morphometry (total, short- and long-arm length) and, consequently, the class differed among the karyotypes of the species. Based on the first karyograms for these Bromus species, some morphologically similar and several distinct chromosome pairs were found. Therefore, the karyotype characterization confirmed the hexaploid origin of the studied Bromus species, which differ in relation to the karyogram and the nuclear 2C value. Considering this, cytogenetics and flow cytometry can be used to discriminate Bromus species, contributing to taxonomy and systematic studies and providing information on the evolutionary history of this taxa.

Karyogram, nuclear genome size, polyploidy, forage grasses

The genus Bromus Linnaeus, 1753, family Poaceae comprises more than 160 species of annual and perennial grasses (Acedo and Liamas 2001). This taxon is widely distributed around the world (Williams et al. 2011), demonstrating the adaptability of its species (Martinello and Schifino-Wittmann 2003). The genus Bromus includes important forage grasses, such as Bromus auleticus Trinius ex Nees, 1829, Bromus brachyanthera Döll, 1878 and Bromus catharticus Vahl, 1791 (Puecher et al. 2001, Martinello and Schifino-Wittmann 2003, Iannone et al. 2010).

The basic chromosome number of the genus Bromus is x = 7, and its species possess karyotypes varying from 2n = 2x = 14 (diploid) to 2n = 12x = 84 (duodecaploid) (Fedorov 1969, Armstrong 1984, 1987, Klos et al. 2009, Williams et al. 2011). Most of the species are diploid (2n = 2x = 14) or tetraploid (2n = 4x = 28) (Martinello and Schifino-Wittmann 2003), but large variation in chromosome number among Bromus species has been found, such as: Bromus cappadocicus Boissier et Balansa, 1857, and B. tomentosus Trinius, 1813, with 2n = 2x = 14; B. erectus Huds., 1762, B. biebersteinii Roemer et Schultes, 1817, and B. stenostachyus Boissier, 1884, with 2n = 4x = 28; B. tomentellus Boissier, 1846, B. variegatus M. Bieberstein, 1819 (Sheidai et al. 2008), B. auleticus (Martinello and Schifino-Wittmann 2003), B. bonariensis Parodi et J. H. Camara, 1963, B. brevis Steudel, 1854, B. parodii Covas et Itria, 1968, B. brachyanthera and B. catharticus Vahl, 1791 (Schifino and Winge 1983, Naranjo 1985) with 2n = 6x = 42. Variation in chromosome number has also been found within the same species, such as in Bromus kopetdagensis Drobow, 1925, (2n = 6x = 42 in Tehran and 2n = 10x = 70 in Emamzadeh-Hashem; Sheidai et al. 2008) and in Bromus setifolius J. Presl, 1830, (2n = 10x = 70 for ‘Pictus’ and ‘Brevifolius’, and 2n = 4x = 28 for ‘Setifolius’; García et al. 2009). So, cytogenetic and plant morphology data supported the classification of the B. setifolius lines as separate species (García et al. 2009).

Karyotype characterization showed that the chromosomes of the Bromus species are similar in relation to total length and class (Joachimiak et al. 2001), with the occurrence of metacentric and submetacentric chromosomes being reported (Sheidai et al. 2008). This way, the distinction between karyotypes of Bromus species with the same chromosome number is generally carried out based on the size of the satellite portions (Armstrong 1983, Joachimiak et al. 2001) and heterochromatin distribution (Klos et al. 2009).

Stebbins (1981) reported that the genus Bromus originated in Eurasia. During the Pliocene, three sections were originated: Ceratochloa P. Beauvois, 1812, Pnigma Dumort, 1823 and Neobromus Shear, 1900, being that Ceratochloa and Neobromus spread to Americas. Given this hypothesis, Stebbins argued that the geographic region Eurasia was also the differentiation center of diploid, tetraploid and, most likely, hexaploid species.

Differentiation of allohexaploid Bromus species in South America proceeded in the Pleistocene. Meanwhile, in North America, allopolyploidy events also occurred, leading to new ones with higher ploidy level (8x, 12x) (Stebbins 1947, 1981). This way, the Bromus species in South America is somewhat restricted to hexaploid species, differently from those found in North America, which are octoploids and duodecaploids (Massa et al. 2004).

According to current knowledge, the South American species (as B. auleticus, B. brachyanthera and B. catharticus) have chromosome number of 2n = 6x = 42 (Schifino and Winge 1983, Naranjo 1985, Martinello and Schifino-Wittmann 2003). Nevertheless, it is still necessary to confirm and understand the chromosomal changes among these species. Because chromosomal changes constitute an important mechanism of diversification and speciation (Stace 2000, Peer et al. 2009, Weiss-Schneeweiss et al. 2013), the investigation of this aspect in Bromus species of South America may generate knowledge on the speciation processes in this genus.

Numerical and structural chromosomal rearrangements have been reported to trigger changes in karyotype in various plant taxa. Due to these changes, the nuclear genome size varies between phylogenetically related species (Raskina et al. 2008, Bonifácio et al. 2012). Thus, nuclear DNA content measurements have increasingly been employed in taxonomic, systematic and evolutive approaches using flow cytometry (FCM). In addition to its practicality and reproducibility, FCM is useful to reveal differences among taxa, especially those that exhibit conserved chromosome number (Mabuchi et al. 2005). The nuclear genome size was measured for Bromus hexaploid (2n = 6x = 42) species. In spite of the same chromosome number, the seven species showed distinct nuclear 2C values, varying from 2C = 12.72 pg in Bromus willdenowii Kunth, 1829, to 2C = 15.10 pg in Bromus lithobius Trin., 1836 (Klos et al. 2009).

Hence, karyotype and nuclear 2C value are relevant data for the taxonomy and systematics of Bromus, as well as for understanding the evolutionary history of the genus and the relationships within the taxa. Thus, the aims of the present study were to measure the nuclear 2C value, determine the chromosome number and characterize the karyotype of the South American Bromus species B. auleticus, B. brachyanthera and B. catharticus.

FCM nuclear suspensions resulted in G₀/G₁ fluorescence peaks with a coefficient of variation of less than 5% for Bromus species and P. sativum. Thereby, FCM procedures provided suspensions with adequate amount of isolated, intact and stoichiometrically stained nuclei. The 2C nuclear DNA content was measured for the Bromus species through analysis of the histograms. 2C value of B. catharticus was 2C = 12.64 ± 0.00 pg, B. brachyanthera was 2C = 16.73 ± 0.16 pg, and B. auleticus was 17.92 ± 0.44 pg. Mean value for B. auleticus was 41.70% higher than for B. catharticus, and 7.10% greater than for B. brachyanthera. In turn, B. brachyanthera presented 2C value 32.36% higher than that of B. catharticus. These values reflect interspecific variation among the nuclear genome sizes of the analyzed species.

Root meristems treated with amiprophos-methyl and macerated in enzyme pool solution resulted in adequate metaphase chromosomes. Metaphases were chosen based on the following criteria: well-spread chromosomes with well-defined constriction, no chromatin deformations and no cytoplasmic background noise. These features allowed accurate chromosome counting, karyotype measurements, chromosome class determination and karyogram assembly. All three Bromus species showed a conserved number of 2n = 42 chromosomes (Figure 1).

Based on the morphometric data, the chromosome class was determined and the differences between the karyotypes of the three species were verified. B. auleticus presented eleven metacentric (1, 3, 4, 5, 6, 8, 9, 11, 15, 18 and 19) and ten submetacentric chromosomes (2, 7, 10, 12, 13, 14, 16, 17, 20 and 21). B. brachyanthera exhibited 13 metacentric (1, 2, 4, 5, 6, 9, 10, 13, 15, 16, 17, 19 and 21) and eight submetacentric chromosomes (3, 7, 8, 11, 12, 14, 18 and 20). Finally, B. catharticus displayed eleven metacentric (1, 4, 5, 7, 8, 9, 10, 11, 14, 17 and 20) and ten submetacentric chromosomes (2, 3, 6, 12, 13, 15, 16, 18, 19 and 21) (Table 1). Groups of morphologically similar chromosome pairs were also identified for all Bromus species: 3–4 in B. auleticus, 11–12 and 15–16 in B. brachyanthera, and 12–13 in B. catharticus (Figure 1, Table 1).

The three Bromus species presented only metacentric and submetacentric chromosomes. Despite belonging to the same class, the chromosomes differed intra- and interspecifically based on their morphology, which was characterized by occurrence of well-defined telomere and centromere portions and relatively low chromatin compaction level (Figure 1). As summarized in Table 1, the majority of the chromosomes could be distinguished by at least one morphometric parameter: total length, short- and long-arm length, ratio between arms, and/or relative chromosome size (%) in relation to the sum of the total length (Table 1). In addition, some chromosomes showed the secondary constriction in the interstitial region of the short arm, such as the chromosome 3 of B. catharticus (Figure 1a) or 18 of B. auleticus (Figure 1c).

Figure 1.

First karyograms of Bromus species, displaying 2n = 6x = 42 chromosomes: a B. catharticus b B. brachyanthera and c B. auleticus. a B. catharticus displayed eleven metacentric (1, 4, 5, 7, 8, 9, 10, 11, 14, 17 and 20) and ten submetacentric chromosomes (2, 3, 6, 12, 13, 15, 16, 18, 19 and 21) b B. brachyanthera exhibited 13 metacentric (1, 2, 4, 5, 6, 9, 10, 13, 15, 16, 17, 19 and 21) and eight submetacentric chromosomes (3, 7, 8, 11, 12, 14, 18 and 20) c B. auleticus presented eleven metacentric (1, 3, 4, 5, 6, 8, 9, 11, 15, 18 and 19) and ten submetacentric chromosomes (2, 7, 10, 12, 13, 14, 16, 17, 20 and 21). Note the morphologically similar chromosomes: a 12–13 in B. catharticus b 11–12 and 15–16 in B. brachyanthera, and c 3–4 in B. auleticus. Bar = 5 µm.

According to the chromosome number found here and the complement set x = 7 (Stebbins 1981), B. auleticus, B. brachyanthera and B. catharticus are hexaploid species (2n = 6x = 42). This result is in accordance with previous data reported for the three species (Schifino and Winge 1983, Naranjo 1985, Martinello and Schifino-Wittmann 2003). The genus Bromus originated in Eurasia, whereas the hexaploid species emerged in South America during the Pleistocene, from the subgenus Ceratochloa (Stebbins 1981). Thus, the chromosome number of the three species has remained conserved in relation to the ancestors, supporting Stebbins’s hypothesis on the diversification of the Bromus species.

The mean nuclear 2C value divergences were corroborated by chromosome morphometry. B. catharticus clearly differed from the other species, being that its relatively small genome size correlated with the sum of the total chromosome length. Differently, for B. auleticus and B. brachyanthera this relation was not observed, as a result of the low compaction level of the chromatin in the latter species (Figure 1, Table 1). The large differences among nuclear 2C values for the same chromosome number suggests that the three species diverged through chromosomal rearrangements. In a study of hexaploid Bromus species, Klos et al. (2009) also reported interspecific variations (2C = 12.72 to 15.10 pg) in relation to the nuclear genome size. According to these authors, chromosomal changes occurred during the evolution of the hexaploid Bromus species, most likely through the gain or loss of highly repeated DNA sequences. Regarding this, karyotype and nuclear genome size should be considered together when comparing Bromus species.

Despite the similar morphology of some chromosome pairs (12–13 in B. catharticus, 11–12 and 15–16 in B. brachyanthera, and 3–4 in B. auleticus; Figure 1, Table 1), differences were found for most pairs of the three hexaploid Bromus species (Figure 1, Table 1). For the genus Bromus, Stebbins (1981) classified all species as allohexaploids. The allopolyploidy in Bromus was also found in Bromus hordeaceus L., 1753, which was classified as an allotetraploid (Ainouche et al. 1999). In that sense, Klos et al. (2009) highlighted that the Bromus section Ceratochloa includes a number of closely related allopolyploid species originated by three ancestors AABBCC.

The nuclear 2C value and karyotype characterization allowed differentiating the three Bromus species, thus contributing to the cytotaxonomy and evolutional understanding in this taxon. As also demonstrated by other authors, these data provide insights about the evolutionary process and diversification of the polyploid Bromus species.

We thank the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq, Brasília – DF, Brazil), the Fundação de Amparo à Pesquisa do Espírito Santo (FAPES, Vitória – ES, Brazil), and the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES, Brasília – DF, Brazil) for financial support. Scholarship holder associated to the project MP1 – 01.15.02.002.05.08 of the Rede Nacional de Recursos Genéticos, Vertente Vegetal, Projeto Componente “Bancos ativos de Germoplasma de Forrageiras”.