Gene Flow and Geographic Variation in Natural Populations of Alnus Acumi1'lata Ssp
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Rev. Biol. Trop., 47(4): 739-753, 1999 www.ucr.ac.cr www.ots.ac.cr www.ots.duke.edu Gene flow and geographic variation in natural populations of Alnus acumi1'lata ssp. arguta (Fagales: Betulaceae) in Costa Rica and Panama OIman Murillo) and Osear Roeha2 Instituto Tecnológico de Costa Rica, Escuela de Ing. Forestal.Apartado 159 7050 Cartago, Costa Rica. Fax: 591-4182. e-mail: omurillo@itcr.. ac.cr 2 Universidad de Costa Rica, Escuela de Biología, Campus San Pedro, San José, Costa Rica. e-mail: [email protected] Received 16-IX-1998. Corrected 05-IV-1999. Accepted 16-IV-1999 Abstract: Seventeen natural populations in Costa Rica andPanama were used to asses geneflow and geographic patternsof genetic variation in tbis tree species. Gene flow analysis was based on the methods of rare alleles and FST (Index of genetic similarity M), using the only four polymorphic gene loci among 22 investigated (PGI-B, PGM-A, MNR-A and IDH-A). The geographic variation analysiswas based on Pearson 's correlations between four geographic and 14 genetic variables. Sorne evidence of isolation by distance and a weak gene flow among geographic regions was found. Patterns of elinal variation in relation to altitude (r = -0.62 for genetic diversity) and latitude (r = -0.77 for PGI-B3) were also observed, supporting the hypothesis of isolation by distance. No privatealleles were found at the single population level. Key words: Alnus acuminata, isozymes, gene flow, eline, geographic variation, Costa Rica, Panarna. Pollen and seed moverrient among counteract any potential for genetic drift subdivided popuIations in a tree speeies, (Hamrick 1992 in Boshieret al. 1995). eounteraet the effeet of genetic divergenee SimilarIy as described by Cordia alliodora promoted by seIeetion and random genetie (Boshier et al. 1995), "from observation during drift within small isoIated popuIations (HartI rain on sloping terrain, the formation of 1980). Spatial distribution, flowering temporary rivulets will lead to additional seed phenology, ineompatibility meehanism, and dispersal and sometimes to c1umped areas of pollen and seed dispersion patterns, all regeneration". Slnce such temporary water eontribute to genetie strueturing of courses carry seed into rivers, dispersal populations. The status of a population at any distances in the order of kilometers arefeasible. moment and its rate of ehange will be the result Such long-distance dispersal events are very of the eombination of all these factors important in coIonization and inbreaking down (Hamrick and Murawski 1991). Alnus , popuIation isoIation vía gene flow. acuminata shows a midsuccessional species Gene flowand its ímportance in evolution pattern, subject to repeated coIonization, has been debated for a long time under a extinction, and recolonization episodes variety of strong víews, as pointed out by associated to natural catastrophies. In such Slatkin (1981): "One view states that gene species, where neighborhood sites may be flow is common and that a small amount of relatively small, interactions among gene flow among different parts of a species' neighborhoods in space and time may range effectively unifies the species and affects 740 REVISTA DE BIOLOGÍA TROPICAL signifieantly the genetie ehanges in eaeh part of spatial adaptation's patterns of enzyme of the range". The other view is that gene phenotypie (genotypic) polymorphisms to flow is uncommon and that natural seleetion gradually varying environmental faetors acts more or less independently in each part (Bergmann 1978, Bergmann and Gregorius of the range of the species. Besides, the 1993), and thus, reflects pnmary estimates of gene flow can be severely intergradation. However, it is important to affected sinee there is always a non notieeable know whether the extent of gene flow between amount of gene flow, as well as, a large populations depends on their geographic amount that may have migrated but not breed. distances, "isolation by distance", as originally Therefore, methods of estimatíng gene flow proposed by Wright (1943 in Yang and Yeh among populations have been proposed and 1995). Therefore, it is intended to elucidate in diseussed by Slatkin (1981, 1985) and found this study the possible patterns of gene flow Jater that FST and rare-alleles methods yield throughout the Alnus acuminata natural comparable estimates under a wide variety of occurrence in Costa Rica and Panama, as well conditions. However, since there are practica! as the possibility of isolation by distance problems in estimating the frequencies of rare among the different geographieal regions. alleles in electrophoretic studies, especially Alnus acuminata is one of the most when sampling diploid genotypes (Gregorius widely distributed species of the genus. It 1980), is therefore FST likely to be more covers a large discontinuous geographical useful under realistic conditions (Slatkin and range throughout the Americas, stretching Barton 1989, Slatkin 1993). from central Mexico to northern Argentina Gene flow is often analyzed under (Lamprecht ] 990). This tree speeies was ecogeographical or elinal patterns of species' recently subdivided into three subspeeies, range distribution. The origin of clinal namely acuminata, arguta and glabrata variation in natural populations has been (Furlow 1979a, 1979b). The speeies oceurs debated since Mayr (1942 in Yang and Yeh through wide ecological gradients, from 1995) postulated a differentiation between around 1400 to 3200 m in elevation zones of primary and secondary intergradation. (Anonyrnous 1995, Furlow 1979a, Mayr defined prímary intergradation as elinal Lamprecht 1990), from 2000 until 5000 mm variation that develops within a continuous in annual precipitatíon (Anonymous 1995, series of populations and secondary Coen 1983, Vargas 1994) and from deep and intergradation as the consequence of junction rieh vo1canie soils (andosols) to shallow, between populations that ha ve beeome heavy-textured, low fertility and poorly differentiated in allopatry. Sorne empírical draíned inceptisols (Anonymous 1995, evidenee suggests that current patterns of Vázquez 1983). These highly variable allele frequency elinal variation reflect environmental conditions c1early enhanee secondary contaet between populations that large differences in the eeologieal eonditions were isolated in refugia during the Pleistocene over short distanees throughout the (post-Pleistocene secondary contact) and distribution of this tree species, which may possessing di fferent prevalent alleJes have promoted an important amount of (espeeially rarealleles) at a gene locus (Gallant genetic variation among populations, with et al. 1993, Hattemer et al. 1993, Konnert and large potential benefits. The distribution of Bergmann 1995). Besides, for several this species in Costa Rica is discontinuous, as european tree speeies, an important genetic it is found along the Central Volcaníc effect has been caused by forest utilization in Mountain Range and the Talamanca the last eenturies (Hattemer, et al. 1993). Mountain Range, where the populations are However, other research suggest that elina! or separated one from another by valleys eeogeographieal variation is mostly the result (Alvarez 1956, Camacho and Murillo 1986). MURILLO & ROCHA: Gene flow and geographic variation in natural populations of Alnus acuminata ssp. 741 MATERIALSAND METHODS polyvinylpyrrolidone (PVP) and 1 % mercaptoethanoL The crude extract was Allozyme data: the 17 sampled soaked into paper WICKS (Whatman populations represent geographicalIy and chromatography paper No.3) and then inserted ecologically the distinct natural regions of into the gels after approximately 10 mino Alnus acuminata ssp. arguta in Costa Rica and Horizontal starch gel electrophoresis was Panama (Anonymous, 1995, Murillo et al. performed routinely with TRIS-histidine and 1993) as shown in Table 1. The various single POULIK systems for the different isozymes populations sampled are distribuited along five analyzed, under a voltage of 18 to 20 V.cm-l in natural population regions. Each single a running distance of approximateiy 10 cm population is represented by a random sampie (see Table 2 for details of the different buffer of 31 to 63 adult trees (with an average of 52 and running condítions routinely applied) and trees) separated by a minimum of 50 m following the general known procedures distance. Different materials from each tree (Couto et al 1991, Liengsiri et al. 1990). were electrophoretically assayed for variation Various combinations of electrode and gel in 10 enzymes systems. The samples were buffers (Cheliak and Pitel 1984, Soltis and hand homogenized in a mortar after the Soltis 1989) were initially tested for their addition of two drops of the following suitability to improve the zyrnograms. The extraction buffer (Murillo 1997) adjusted with staining of enzymes was performed according IN HCL to pH 7.3: 0.13 M TRIS; 0.4 mM to recipes given by Cheliak and Pitel (1984) Titriplex II; 0.3 mM dithiothreitol (DTT); 4% and Soltis and Soltis (1989). The enzyme TABLE 1 Natural populations investigated of Alnus aCWllínala ssp. arguta (Schlectendal) Furlow from Costa Rica and Panama RegionIPopulation Range of elevation Latitude Longitude (m.a.s.!.) Region lPoás 1. Zarcero 1700-1800 10° 14' 84° 22' 2. Bajos del Toro 1400-1500 10° 13' 84° 19' 3. Vara Blanca 1700-1800 10° 10' 84° 07' 4. Los Cartagos 2000-2100 10° 9' 84° 09' Region Irazú-Thrrialba 5. Coronado 1400 m.a.s.1. 1300-1400 9° 59' 84° 00' 6. Coronado> 2000 m.a.s.l. 2000-2500 9° 58' 83° 56' 7. Llano Grande 1700-1900 9° 55' 83° 56' 8. lrazú 2900 9° 57' 83° 49' 9. Pacayas 1700-1800 9° 55' 83° 48' IO.Turrialba 2500-2600 9° 58' 83° 47' Regioll Talamailca ¡ l. El Empalme 2100-2200 9° 45' 83° 57' 12. Caón 2300-2400 9° 42' 83° 54' 13. Copey 1700-1900 9° 39' 83° 55' 14. San Gerardo 2000-2200 9° 33' 83° 49' 15. Siberia 2700-2800 9° 33' 83° 41' 16. División 2000-2200 9° 31' 83° 42' Regioill Boquete, Panama 17 Boquete 1400-1600 8° 45' 82° 20' Range 1300-2900 8° 45'-10°14' 82° 20' _84° 22' 742 RE VISTA DE BI OLO GÍA TROPICAL TAB LE 2 Buffers and running condítíons routínely performedfor starch gel electrophoresís of enzymes ín Alnus acuminata ssp.