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Biology Vol. 4, 2003
An outline of our current knowledgeon Mexicancycads(Zamiaceae, Cycadales) Andrew P. Vovidesl,., Miguel Angel Perez-Farrera2,Jorge Gonzales-Astorgal,DoloresGonzalezl, Tim Gregory3, Jeff Chemnic~, Carlos IElesiasl, Pablo Octavio-Aguilar, and Silvia Salas-Morales
Sergio Avendafiol,
Cristina Bareenasl
IInstituto de Ecologia, A. C., Apdo Postal 63, Xalapa, Veracruz,91000 Mexico. 2Escuelade Biologia, Universidad de Ciencias y Artes del Estado de Chiapas, Calz. Samuel Le6n Brindis 15I, Tuxtla Gutierrez, Chiapas,29000 Mexico. 3Mongomery Botanical Center c/o I DNA Way, South San Francisco,CA, USA 94080.4114 Conejo Rd., SantaBarabara,CA, USA. 5SERBO,A. C., A. P. 533, Oaxaca.Oax., Mexico
ABSTRACT During recentyearsresearchon cycadshastakenon an encouragingdynamismworld-wideandmanynew taxaarebeingdiscoveredanddescribed.Thereareca 276 speciesknown to date and many researchers believethe numbermayreachasmanyas400 species when all potential cycad habitats have been investigatedandtaxonomicstudiesarecompleted.In Mexico there are ca 45 speciesdescribedand this figure may increaseto ca 50 in the nearfuture. Here, an outline on currentresearchon Mexicancycadsis given from varying disciplinessuchas morphology, anatomy, taxonomy, ecology, pollination biology, populationgenetics,descriptivephytogeographyand molecularsystematics.Althoughcycadshavea long fossil history dating back to the Permian, recent researchis indicating that most moderntaxa are of relatively recentspeciationand their distribution, at least in Mexico, appear to give support to the Cenozoicfloristic refugetheory. This appearsto be the case for the genus Ceratozamiawherecertain speciescomplexesoccur in southernMexico within Pleistocenefloristic refugesas the centre of origin with subsequentmigrations towards the northeast during climate warming. A similar patternseemsto be evident in the genus Dioon with a centre of speciation in southern Mexico (Oaxaca) with 'e-mail:
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subsequentmigrations up valleys and canyonsand towards the northwest (D. tomase//ii) along the Mexican Pacific with a morphologically distinct specl'es(D. sonorense)in the extremenorthwestern limit. Like-wise for Dioon edu/ewherethe genetic variation was found to be maximum in the southeasternlimit of its rangeandminimumin populations that are morphologically distinct (D. edu/e var. angustifo/ium)in the extremenorthernlimit. Finally, as a result of demographicstudies of Cycads,an integratedconservationstrategyhas beendeveloped for the sustainablemanagement of cycadpopulations by rural communities. INTRODUCTION. What are cycads? --Cycads are an ancient group of dioecious seed plants, classified with the gymnosperms with a fossil history dating back to the Permian and perhaps the Carboniferous [1-4]. Though cycads (sensu lata) were widely distributed during the Mesozoic, their numbers and distribution became drastically reduced during the mass extinction that marked the end of the Cretaceous period. This saw the total extinction of the Cycadeoids (formerly Bennettitales) leaving a reduced number in the Cycadales to survive, thus giving rise to the living cycads of today, which are restricted to tropical and subtropical ecosystems of the world's continents including certain islands. Along with Ginkgo cycads are the only living seed plants that have motile
160 antherozoids [5] a character considered ancestral [4]. The Permianfossil Crossozamia[3] a putativecycad ancestor, shows Cycas-like megasporophylls and strobili, although more delicate than in the extant Cycas, the primitive "seed-fern" type of cycad "seed-leaf' is unquestionable [4]. Two lines of megastrobilus evolution from ancestral Crossozamia can be envisaged. The first has a short cone axis and long megasporophylls bearing a number of marginal ovules directly giving rise to the open and loose Cycas type megastrobilus; and the second, through the JurassicBeania that has a long cone axis and short megasporophylls bearing two ovules on a peltate distal end giving rise to the tight cone-like Zamia type megastrobilus [3]. The extant cycads consist of II genera within three families, all of which appear to be monophyletic [6-8]. Their habit varies from herbaceous, occupying the herbaceous and shrubby forest layers, to arborescent reaching up to 18 m tall such as the Cuban Microcycas calocoma (Miq.) A. DC. One species, Zamia pseudoparasitica Yates is epiphytic on tropica rainforest trees in Panama. Distribution and number:Cycads are presently distributed within subtropical and tropical regions of the world with ca 276 species [9,10]. Many researchers believe the number may reach as many as 400 species when all potential cycad habitats have been investigated and taxonomic studies are completed (Osborne Pers. com.). A recent workshop on cycad classification concepts set out guidelines for future work on cycads, especially the species concept within the group [II]. Countries particularly rich in cycad flora are Australia ca.78 spp, Mexico ca. 45 spp, and South Africa ca. 40 spp. [12]. In the neotropics, their habitats vary from evergreen tropical rainforests, through seasonally dry tropical forests, cloud forests, oak and pine-oak forests, grasslands,savannahsand riverine communities such as the Colombian Zamia chiRUaSeeman. .
Andrew P. Vovides et at.
2-amino-3- (methylamino )-propanoic acid (BMAA) are considered apomorphies to the cycad clade of the seed plants [33]. Cycasin has been reported as a naturally occurring hepatotoxin and carcinogen that can cause chromosome breakage in onion root tips [34,35]. There is also increasing evidence that BMAA toxicity may be the cause of the high incidence of amyotrophic lateral sclerosis/parkinsonism-dementia (ALS/PDC) a neurodegenerative disease common on the island of Guam and known there as "litigo bodig" [4,13,36]. Pollination studies:-- Wind pollination was thought to be the pollination process for cycads, dogma largely established as a result of observation of wind pollination in conifers and extrapolation to cycads, since the male cones of large arborescentcycads like Dioon, Cycas and Encepha/artos produce copious amounts of pollen has been responsible for this [37]. However, the aerodynamics of female cycad cones renders them improbable candidates for wind pollination [38]. Though insects associated with cycad cones have been observed in the past [39] it was not until the work of Norstog et al. [40], Norstog [41], Tang [42] and Norstog & Fawcett [43] that pollination by insects (Curculionidae and Languriidae) had been established for tWft species of Zamia. Entomophily has been postulated for other cycad species by evidence such as volatile emissions from the male cones, differential starch content between male and female cones and idioblast breakdown in female cones prior to receptivity [44-47] as well as by phylogenetic inference in the case of the Cuban Microcycas ca/ocoma (Miq.) A.DC. [46]. Though it may be postulated that the insect/cycad relationship may the result of an old Mesozoic coevolution, molecular evidence however, suggests that the extant cycad species have evolved relatively more recently [48] and this may also be the case for their insect pollinators [49].
CYCADS IN MEXICO MODERN REASEARCH ON CYCADS Owing to renewed interest in cycads during the last 25 years or so, the world cycad flora has increased in number of species after a long period of relative stagnation, after the pioneering work of Charles Joseph Chamberlain during the early XX century [4]. Modern work has been done in the fields of toxicity [13,14] population ecology [15-20], genetics and evolution [2124], cytotaxonomy [25-32]. Cycad toxicity:-- Particularly outstanding is the toxicity of cycads. Certain toxins are peculiar only to cycads, such as cycasin, macrozamin and the neurotoxin
Mexico is particularly rich in cycadswith a high endemismand we considerMexico to be a centreof diversity for the groupin the neotropics.Much of our knowledgeof the distribution of cycadsin Mexico is due to the early exploratorywork of Chamberlain[50]. In recentyearsinformationhas beengreatly expanded by workers from Mexico and Italy [4,51-54] seealso Balduzziet al. [55], Moretti et aI. [56], Stevenson et al. [57] and Moretti & Sabato[58f Three generaare present: Ceratozamia Brongn. (ca. 21 spp), Dioon Lindley (ca. 13 spp) and Zamia L. (ca. 12 spp). Ceratozamiaran~esfrom Mexico into CentralAmerica,
An outline of our current knowledge on Mexican cycads (Zamiaceae, Cycadales)
present in Guatemala, Belize and Honduras (Figure 1). The genus Dioon, largely Mexican, is vicariant only in Honduras and possibly Nicara~ua. Zamia ranlres widely.
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from Florida in the USA through Mexico, all of Central America, the West Indies and into South America, where the genus is particularly diverse in Colombia. Since the geographical distribution of organisms is independent of political frontiers, this needs to be taken into account in order to report on true endemisms based on physiographic, climatic and other factors [59). We can therefore confidently say that the genera Dioon and Ceratozamia are endemic to "Mega-Mexico 2". The term "Mega-Mexico 2", coined by Rzedowski [59) is the extension of the country's frontier into Central America to include northern Nicaragua considering the aforementioned factors. TRF. ~F,NIJ~ CRRATOZAMTA The genus Ceratozamia or "horned" Zamia (K£p
