Wild and agronomically important Agave species (Asparagaceae) show proportional increases in chromosome number, genome size, and genetic markers with increasing ploidy MANUEL L. ROBERT 1 *†, K. YOONG LIM 2 †, LYNDA HANSON 3 , FILIPE SANCHEZ-TEYER 1 , MICHAEL D. BENNETT 3 , ANDREW R. LEITCH 2 and ILIA J. LEITCH 3 1 Unidad de Biotecnologia, Centro de Investigacion Cientifica de Yucatán, A.C. C43 # 130 Col. Chuburná de Hidalgo, Yucatán, México 2 School of Biological and Chemical Sciences, Queen Mary, University of London, Mile End Road, London, E1 4NS, UK 3 Jodrell Laboratory, Royal Botanic Gardens, Kew, Richmond, Surrey, TW9 3DS, UK Received 27 November 2007; accepted for publication 7 February 2008 Agave (Asparagaceae) includes cultivated and wild varieties of henequen used for hard fibre production. As part of a breeding programme to improve Agave production, species with different ploidy levels were genetically character- ized: two diploids [A. tequiliana Weber and the hybrid H11648 ((A. amaniensis Trel. & Nowell ¥ A. angustifolia Haw.) ¥ A. amaniensis)], a triploid (A fourcroydes Lem. var. kitam ki), a tetraploid (A. angustifolia var. letona), three pentaploids (A. fourcroydes var. sac ki, A. fourcroydes var. yaax ki, and A. sisalana Perrine), and two hexaploids (A. angustifolia var. chelem ki from two locations). Chromosome spreading was used to determine the chromosome number, flow cytometry was employed to measure the genome size, and fluorescent in situ hybridization was performed using 45S and 5S ribosomal DNA (rDNA) and the telomeric sequences (TTAGGG)n and (TTTAGGG)n as genetic markers. There were proportional increases with ploidy level of the following: (1) chromosome number (from diploid 2n = 2x = 60 to hexaploid 2n = 6x = 180), including the number of large and small chromosomes in the bimodal karyotype of Agave; (2) genome size, with a mean monoploid genome size (1Cx) of 7.5 pg (range, 7.36–7.61 pg); and (3) the number and distribution of 45S and 5S rDNA loci, with one locus of each per basic, monoploid genome. Thus there was complete additivity in genome structure with increasing ploidy, as reported in some angiosperm polyploids. However, as other analyses of polyploids have revealed a decrease in 1Cx values with increased ploidy, possible explanations for the observed genomic stability were considered. With the (TTAGGG)n probe, the signal was localized at the telomeres, consistent with published data showing that many species in the order Asparagales have this type of telomere sequence. It is speculated that sporadic telomeric signals using the (TTTAGGG)n probe are probably derived from either errors in telomerase activity or relic ancestral-type telomeric sequences. © 2008 The Linnean Society of London, Botanical Journal of the Linnean Society, 2008, 158, 215–222. ADDITIONAL KEYWORDS: bimodal karyotype – C value – flow cytometry – fluorescence in situ hybrid- ization – genome additivity – henequen – polyploidy – ribosomal DNA – telomere. INTRODUCTION The genus Agave comprises c. 200 species with the centre of diversity located in Mexico (García- Mendoza, 2002). Many of these species are of great economic importance, including A. tequilana, which is the raw material for the manufacture of alcoholic beverages, and A. fourcroydes, which is used for hard fibre production and steroidal bioactive compounds. The genus contains species of many ploidy levels (2x, 3x,4x,5x,6x, and 8x) with a basic chromosome number of n = 30. *Corresponding author. E-mail: robert@cicy.mx †Joint first authors. Botanical Journal of the Linnean Society, 2008, 158, 215–222. With 1 figure © 2008 The Linnean Society of London, Botanical Journal of the Linnean Society, 2008, 158, 215–222 215 Downloaded from https://academic.oup.com/botlinnean/article-abstract/158/2/215/2418219 by guest on 31 May 2020