Abstract
Agave Linnaeus, 1753 is endemic of America and is considered one of the most important crops in Mexico due to its key role in the country’s economy. Cytogenetic analysis was carried out in Agave tequilana Weber, 1902 ‘Azul’, Agave cupreata Trelease et Berger, 1915 and Agave angustifolia Haworth, 1812. The analysis showed that in all species the diploid chromosome number was 2n = 60, with bimodal karyotypes composed of five pairs of large chromosomes and 25 pairs of small chromosomes. Furthermore, different karyotypical formulae as well as a secondary constriction in a large chromosome pair were found in all species. Fluorescent in situ hybridization (FISH) was used for physical mapping of 5S and 18S ribosomal DNA (rDNA). All species analyzed showed that 5S rDNA was located in both arms of a small chromosome pair, while 18S rDNA was associated with the secondary constriction of a large chromosome pair. Data of FISH analysis provides new information about the position and number of rDNA loci and helps for detection of hybrids in breeding programs as well as evolutionary studies.
Highlights
Agave Linnaeus, 1753 is a genus of the monocotyledonous family Asparagaceae, belonging to the subfamily Agavoideae (APGIII 2009)
Plant ribosomal DNA (rDNA) consists of the 18S, 5.8S and 26S (45S) and 5S genes; in yeasts, these genes are juxtaposed in the same locus, whereas in higher eukaryotes, they are organized as families of tandemly repeated units located at one or a few chromosomal sites (Lavania et al 2005, Garcia et al 2009). 45S ribosomal RNA (rRNA) genes are clustered in tandem arrays of repeat units of 18S, 5.8S and 26S genes, internal transcribed spacers (ITS) and external non-transcribed spacers (NTS), with an approximate size of 7.5–18.5 Kb in plants (Mizuochi et al 2007). 5S rRNA genes occur in high numbers as tandem repeats, usually independent of 45S rDNA, co-localization of 45S and 5S rDNA have been reported in some angiosperms as Silene chalcedonica E.H.L
Fluorescent in situ hybridization (FISH) experiments with both probes labeled with biotin and detected as a red signals, showed that the number of sites of rDNA were constant among all the species under study. 5S rDNA loci were located in both arms of small chromosome pair in each species (Fig. 1)
Summary
Agave Linnaeus, 1753 is a genus of the monocotyledonous family Asparagaceae, belonging to the subfamily Agavoideae (APGIII 2009). 5S rRNA genes occur in high numbers as tandem repeats, usually independent of 45S rDNA, co-localization of 45S and 5S rDNA have been reported in some angiosperms as Silene chalcedonica E.H.L. Krause, 1901 (Siroky et al 2001) and Artemisia Linnaeus, 1753 (Garcia et al 2007); 5S rDNA repeat unit size ranges between 0.2-0.9 Kb, with a highly conserved region (120 bp in length) separated by a NTS (Specht et al 1997). These genes are highly conserved, so they have been used as molecular markers in a large number of plant species, such as Triticum Linnaeus, 1753 (Jiang and Gill 1994), Gossypium hirsutum Linnaeus, 1763 (Ji et al 1999), Hordeum vulgare Linnaeus, 1753 ‘Plaisant’ (Cuadrado and Jouve 2010); comparative studies using rDNA as markers in Agave have been limited, such as those by Robert et al (2008), where they reported the number of rDNA loci in a few species and demonstrated the existence of additivity in the number of loci with increasing ploidy
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