Spatial Distribution Of Genotypes Research Articles

Spatial distribution of conspecific genotypes within chimeras of the branching coral Stylophora pistillata

Chimerism is a coalescence of conspecific genotypes. Although common in nature, fundamental knowledge, such as the spatial distribution of the genotypes within chimeras, is lacking. Hence, we investigated the spatial distribution of conspecific genotypes within the brooding coral Stylophora pistillata, a common species throughout the Indo-Pacific and Red Sea. From eight gravid colonies, we collected planula larvae that settled in aggregates, forming 2–3 partner chimeras. Coral chimeras grew in situ for up to 25 months. Nine chimeras (8 kin, 1 non-related genotypes) were sectioned into 7–17 fragments (6–26 polyps/fragment), and genotyped using eight microsatellite loci. The discrimination power of each microsatellite-locus was evaluated with 330 ‘artificial chimeras,’ made by mixing DNA from three different S. pistillata genotypes in pairwise combinations. In 68% of ‘artificial chimeras,’ the second genotype was detected if it constituted 5–30% of the chimera. Analyses of S. pistillata chimeras revealed that: (a) chimerism is a long-term state; (b) conspecifics were intermixed (not separate from one another); (c) disproportionate distribution of the conspecifics occurred; (d) cryptic chimerism (chimerism not detected via a given microsatellite) existed, alluding to the underestimation of chimerism in nature. Mixed chimerism may affect ecological/physiological outcomes for a chimera, especially in clonal organisms, and challenges the concept of individuality, affecting our understanding of the unit of selection.

Red foxes harbor two genetically distinct, spatially separated Echinococcus multilocularis clusters in Brandenburg, Germany

BackgroundAlveolar echinococcosis (AE) is a clinically serious zoonosis caused by the fox tapeworm Echinococcus multilocularis. We studied the diversity and the distribution of genotypes of E. multilocularis isolated from foxes in Brandenburg, Germany, and in comparison to a hunting ground in North Rhine-Westphalia.MethodsEchinococcus multilocularis specimens from 101 foxes, 91 derived from Brandenburg and 10 derived from North Rhine-Westphalia, were examined. To detect potential mixed infections with different genotypes of E. multilocularis, five worms per fox were analyzed. For genotyping, three mitochondrial markers, namely cytochrome c oxidase subunit 1 (Cox1), NADH dehydrogenase subunit 1 (Nad1), and ATP synthase subunit 6 (ATP6), and the nuclear microsatellite marker EmsB were used. To identify nucleotide polymorphisms, the mitochondrial markers were sequenced and the data were compared, including with published sequences from other regions. EmsB fragment length profiles were determined and confirmed by Kohonen network analysis and grouping of Sammon’s nonlinear mapping with k-means clustering. The spatial distribution of genotypes was analyzed by SaTScan for the EmsB profiles found in Brandenburg.ResultsWith both the mitochondrial makers and the EmsB microsatellite fragment length profile analyses, mixed infections with different E. multilocularis genotypes were detected in foxes from Brandenburg and North Rhine-Westphalia. Genotyping using the mitochondrial markers showed that the examined parasite specimens belong to the European haplotype of E. multilocularis, but a detailed spatial analysis was not possible due to the limited heterogeneity of these markers in the parasite population. Four (D, E, G, and H) out of the five EmsB profiles described in Europe so far were detected in the samples from Brandenburg and North Rhine-Westphalia. The EmsB profile G was the most common. A spatial cluster of the E. multilocularis genotype with the EmsB profile G was found in northeastern Brandenburg, and a cluster of profile D was found in southern parts of this state.ConclusionsGenotyping of E. multilocularis showed that individual foxes may harbor different genotypes of the parasite. EmsB profiles allowed the identification of spatial clusters, which may help in understanding the distribution and spread of the infection in wildlife, and in relatively small endemic areas.Graphical

Mycobacterium tuberculosis complex genotypes circulating in Nigeria based on spoligotyping obtained from Ziehl-Neelsen stained slides extracted DNA.

Tuberculosis (TB) remains one of the most threatening diseases and Nigeria has one of the world’s largest burdens. This study performed high-throughput spoligotyping directly on sputum smears to describe the Mycobacterium tuberculosis strains circulating among new TB patients in Nigeria.MethodsAll State TB control programmes in Nigeria were requested to submit 25–50 smear-positive Ziehl-Neelsen (ZN) stained slides for screening during 2013–2014. DNA was extracted from 929 slides for spoligotyping and drug-resistance analysis using microbead-based flow-cytometry suspension arrays.ResultsSpoligotyping results were obtained for 549 (59.1%) of 929 samples. Lineage 4 Cameroon sublineage (L4.6.2) represented half of the patterns, Mycobacterium africanum (L5 and L6) represented one fifth of the patterns, and all other lineages, including other L4 sublineages, represented one third of the patterns. Sublineage L4.6.2 was mostly identified in the north of the country whereas L5 was mostly observed in the south and L6 was scattered. The spatial distribution of genotypes had genetic geographic gradients. We did not obtain results enabling the detection of drug-resistance mutations.Conclusion/SignificanceWe present the first national snapshot of the M. tuberculosis spoligotypes circulating in Nigeria based on ZN slides. Spoligotyping data can be obtained in a rapid and high-throughput manner with DNA extracted from ZN-stained slides, which may potentially improve our understanding of the genetic epidemiology of TB.

Genetic structure of the old black poplar population along the bank of the Vistula River in Poland

Black poplar (<em>Populus nigra</em> L.) is one of the main woody riparian species in Europe. Because of extensive habitat loss due to river regulations, this species is considered rare and threatened. To analyze genetic diversity and spatial genetic structure, we examined ten nuclear microsatellite loci in a population of very old <em>P. nigra</em> trees growing along the Vistula River in Poland. We found a high level of genetic diversity (<em>H</em><span><sub>E</sub></span> = 0.792, <em>H</em><span><sub>O</sub></span> = 0.731, <em>A</em> = 14.7) that was within the range of other natural European <em>P. nigra</em> populations, and our results showed that sexual propagation is the dominant way of reproduction in the studied population, leading to high clonal diversity (<em>R</em> = 0.91). Additionally, we did not detect a spatial genetic structure resulting in a random spatial distribution of genotypes. Individuals from such old and diverse populations have the potential to provide valuable reproductive material for both restoration programs and breeding purposes.

Using molecular epidemiology to track Toxoplasma gondii from terrestrial carnivores to marine hosts: implications for public health and conservation.

BackgroundEnvironmental transmission of the zoonotic parasite Toxoplasma gondii, which is shed only by felids, poses risks to human and animal health in temperate and tropical ecosystems. Atypical T. gondii genotypes have been linked to severe disease in people and the threatened population of California sea otters. To investigate land-to-sea parasite transmission, we screened 373 carnivores (feral domestic cats, mountain lions, bobcats, foxes, and coyotes) for T. gondii infection and examined the distribution of genotypes in 85 infected animals sampled near the sea otter range.Methodology/Principal FindingsNested PCR-RFLP analyses and direct DNA sequencing at six independent polymorphic genetic loci (B1, SAG1, SAG3, GRA6, L358, and Apico) were used to characterize T. gondii strains in infected animals. Strains consistent with Type X, a novel genotype previously identified in over 70% of infected sea otters and four terrestrial wild carnivores along the California coast, were detected in all sampled species, including domestic cats. However, odds of Type X infection were 14 times higher (95% CI: 1.3–148.6) for wild felids than feral domestic cats. Type X infection was also linked to undeveloped lands (OR = 22, 95% CI: 2.3–250.7). A spatial cluster of terrestrial Type II infection (P = 0.04) was identified in developed lands bordering an area of increased risk for sea otter Type II infection. Two spatial clusters of animals infected with strains consistent with Type X (P≤0.01) were detected in less developed landscapes.ConclusionsDifferences in T. gondii genotype prevalence among domestic and wild felids, as well as the spatial distribution of genotypes, suggest co-existing domestic and wild T. gondii transmission cycles that likely overlap at the interface of developed and undeveloped lands. Anthropogenic development driving contact between these cycles may increase atypical T. gondii genotypes in domestic cats and facilitate transmission of potentially more pathogenic genotypes to humans, domestic animals, and wildlife.

Spatial genetic structure reflects extensive clonality, low genotypic diversity and habitat fragmentation in Grevillea renwickiana (Proteaceae), a rare, sterile shrub from south-eastern Australia.

The association of clonality, polyploidy and reduced fecundity has been identified as an extinction risk for clonal plants. Compromised sexual reproduction limits both their ability to adapt to new conditions and their capacity to disperse to more favourable environments. Grevillea renwickiana is a prostrate, putatively sterile shrub reliant on asexual reproduction. Dispersal is most likely limited by the rate of clonal expansion via rhizomes. The nine localized populations constituting this species provide an opportunity to examine the extent of clonality and spatial genotypic diversity to evaluate its evolutionary prospects. Ten microsatellite loci were used to compare genetic and genotypic diversity across all sites with more intensive sampling at four locations (n = 185). The spatial distribution of genotypes and chloroplast DNA haplotypes based on the trnQ-rps16 intergenic spacer region were compared. Chromosome counts provided a basis for examining genetic profiles inconsistent with diploidy. Microsatellite analysis identified 46 multilocus genotypes (MLGs) in eight multilocus clonal lineages (MLLs). MLLs are not shared among sites, with two exceptions. Spatial autocorrelation was significant to 1·6 km. Genotypic richness ranged from 0 to 0·33. Somatic mutation is likely to contribute to minor variation between MLGs within clonal lineages. The eight chloroplast haplotypes identified were correlated with eight MLLs defined by ordination and generally restricted to single populations. Triploidy is the most likely reason for tri-allelic patterns. Grevillea renwickiana comprises few genetic individuals. Sterility has most likely been induced by triploidy. Extensive lateral suckering in long-lived sterile clones facilitates the accumulation of somatic mutations, which contribute to the measured genetic diversity. Genetic conservation value may not be a function of population size. Despite facing evolutionary stagnation, sterile clonal species can play a vital role in mitigating ecological instability as floras respond to rapid environmental change.

Pollen limitation and reduced reproductive success are associated with local genetic effects in Prunus virginiana, a widely distributed self-incompatible shrub

A vast quantity of empirical evidence suggests that insufficient quantity or quality of pollen may lead to a reduction in fruit set, in particular for self-incompatible species. This study uses an integrative approach that combines field research with marker gene analysis to understand the factors affecting reproductive success in a widely distributed self-incompatible species, Prunus virginiana (Rosaceae). Twelve patches of P. virginiana distributed within three populations that differed in degree of disturbance were examined. Two of the sites were small (7-35 km(2)) remnants of forest in an intensively used agricultural landscape, while the third was continuous (350 km(2)) and less disturbed. Field studies (natural and hand cross-pollinations) were combined with marker gene analyses (microsatellites and S-locus) in order to explore potential factors affecting pollen delivery and consequently reproductive success at landscape (between populations) and fine scales (within populations). Reductions in reproductive output were found in the two fragments compared with the continuous population, and suggest that pollen is an important factor limiting fruit production. Genetic analyses carried out in one of the fragments and in the continuous site suggest that even though S-allele diversity is high in both populations, the fragment exhibits an increase in biparental inbreeding and correlated paternity. The increase in biparental inbreeding in the fragment is potentially attributable to variation in the density of individuals and/or the spatial distribution of genotypes among populations, both of which could alter mating dynamics. By using a novel integrative approach, this study shows that even though P. virginiana is a widespread species, fragmented populations can experience significant reductions in fruit set and pollen limitation in the field. Deatiled examination of one fragmented population suggests that these linitations may be explained by an increase in biparental inbreeding, correlated paternity and fine-scale genetic structure. The consistency of the field and fine-scale genetic analyses, and the consistency of the results within patches and across years, suggest that these are important processes driving pollen limitation in the fragment.

Investigating the effects of topography and clonality on genetic structuring within a large Norwegian population of Arabidopsis lyrata

The gene flow through pollen or seeds governs the extent of spatial genetic structure in plant populations. Another factor that can contribute to this pattern is clonal growth. The perennial species Arabidopsis lyrata ssp. petraea (Brassicaceae) is a self-incompatible, clonal species found in disjunctive populations in central and northern Europe. Fourteen microsatellite markers were employed to study the level of kinship and clonality in a high-altitude mountain valley at Spiterstulen, Norway. The population has a continuous distribution along the banks of the River Visa for about 1.5 km. A total of 17 (10 m x 10 m) squares were laid out in a north-south transect following the river on both sides. It is shown that clonal growth is far more common than previously shown in this species, although the overall size of the genets is small (mean diameter = 6.4 cm). Across the whole population there is no indication of isolation by distance, and spatial genetic structure is only visible on fine spatial scales. In addition, no effect of the river on the spatial distribution of genotypes was found. Unexpectedly, the data show that populations of small perennials like A. lyrata can behave like panmictic units across relatively large areas at local sites, as opposed to earlier findings in central Europe.

Microsatellite loci for a new Australian endemic plant species Erythroxylum sp. ‘Cholmondely Creek’ (J.R.Clarkson 9367) (Erythroxylaceae)

A population of a recently discovered Australian endemic plant species in the genus Erythroxylum (Erythroxylaceae), previously thought only to occur on the Cape York Peninsula, was found at Gove in the Northern Territory. As the only population in the Northern Territory it is likely to be listed as critically endangered within the state. Unlike the other populations it was found to produce mainly vegetatively. To investigate levels of clonality in comparison to non-clonal populations and the spatial distribution of genotypes we developed microsatellite markers. Following an enrichment protocol 10 loci were found to be polymorphic after screening twenty plants from a sexually reproducing population. Number of alleles varied from 3 to 11 while expected heterozygosity ranged between 0.14 and 0.84. While these markers will form a valuable part of an ongoing investigation into the conservation of the isolated population, successful cross species amplification indicates their potential use in other studies.

Fixation of alleles and depleted levels of genetic variation within populations of the endangered lithophytic orchid Amitostigma gracile (Orchidaceae) in South Korea: implications for conservation

To date, empirical studies on levels of genetic diversity in highly isolated populations of lithophytic orchids are few. We investigated levels of allozyme diversity and spatial distribution of genotypes in small, isolated populations of the endangered, self-compatible lithophyte Amitostigma gracile from southern Korea. At the regional level, genetic diversity was extremely low within nine populations (mean expected heterozygosity, H e = 0.005); among 20 putative loci screened, only one locus (Dia-1) was polymorphic. At this locus Dia-1 a was fixed in two populations, whereas Dia-1 b was in four populations, which as well as the scarcity of genetic variation might be indicative of random genetic drift. Similarly, at the landscape level, Dia-1 a was fixed in a population, whereas Dia-1 b with a high frequency (0.917) was detected in another population (two populations were separated by only 150 m). Analysis of fine-scale genetic structure within populations revealed significant declines in kinship with distance, suggesting that most seeds fall around maternal plants. Furthermore, a significantly high degree of population differentiation and a considerable deficit of heterozygosity were found among and within populations (F ST = 0.857; F IS = 0.940). These results suggest that genetic drift as a consequence of a small effective population size (genetic bottleneck), coupled with a limited gene flow would be major factors in harboring extremely low levels of genetic variation and in the shaping of population genetic structure of A. gracile in South Korea.

Regeneration mode affects spatial genetic structure of Nothofagus dombeyi forests

Disturbance may generate population bottlenecks by reducing population size and the number of founders establishing a new colony. We tested the hypothesis that the scale of disturbance affects the levels of genetic diversity and the spatial distribution of genotypes in naturally regenerating stands of Nothofagus dombeyi, an evergreen angiosperm tree, in northwestern Patagonia. At similar spatial scales, we predicted that old-growth stands characterized by fine-scale gap phase dynamics would be genetically diverse due to restricted gene flow among temporal and spatially isolated gaps. In contrast, young massively regenerated postfire cohorts resulting from coarse-scale disturbances would be genetically more homogeneous. At each of three paired old-growth and postfire stands a minimum of 50 trees were mapped and sampled within 1 ha. Fresh tissue was collected for isozyme analysis from a total of 361 trees along with tree cores and diameters. Tree age distributions reflected the dominant modes of regeneration. Six out of nine analysed loci were polymorphic. Mean genetic diversity parameters were greater but not significant in mature stands. Fixation indices suggested significant heterozygous deficit at two-thirds of possible tests indicating a Wahlund effect due to local recruitment of related seeds. F(ST) indicated moderate between-stand divergence. Mature stands concentrated half of positively like joins and yielded significant (P < 0.05) autocorrelation coefficients at small distance classes (< 20 m). Fine-scale patch dynamics within mature stands favours the maintenance of fine-scale genetic structure as a result of shade intolerance and local seed dispersal. Conversely, postfire stands suffer the effects of genetic drift given that a few reproductive trees produce a somewhat impoverished and genetically uniform progeny. Bottleneck effects will depend upon the density of remnant trees which could also be a function of the severity of fire.

The statute of limitations

Eileen is a professor at Enormous State University. Her recent work on coral populations has been fruitful and she can hardly find the time to follow up on all her ideas. ESU has an informal brown bag seminar series in which graduate students and faculty present and critique ideas and data that are formative or in-progress. A year ago, Eileen presented an intriguing new idea: a Genetic Equilibrium Model, which she referred to as GEM. Bill, one of her departmental colleagues, was enthusiastic about the concept. Bill had just returned from a sabbatical with reams of data on the spatial distribution of genotypes in bracket fungi. Bill thought that GEM would be a great explanation for some of the patterns in his data. Bill approached Eileen afterwards and offered to collaborate by publishing GEM along widi tests of the model using his data. Eileen politely declined. She wanted to test it herself, and she expected that someone in her lab would pursue the idea soon. A year after her seminar, Eileen hasn't published GEM and all of her stu? dents are busy with other projects. Bill and Eileen are discussing it in Eileen's office.

Fine-scale genetic structure of grape phylloxera from the roots and leaves of Vitis.

Patterns of variation at microsatellite loci suggest that root populations of the pest grape phylloxera (Daktulosphaira vitifoliae) are largely parthenogenetic in Australian vineyards. To investigate reproduction in leaf galling phylloxera and the association between these individuals and phylloxera on roots, we examined in detail genetic variation in phylloxera from a vineyard block. Some genotypes found on leaf galls within this block were not present on roots, whereas others spanned both zones. There was no evidence that genotypes on roots were the product of sexual reproduction in leaf galls. mtDNA variation was not associated with the location of the phylloxera clones. The spatial distribution of genotypes within a root population was further investigated by intensively sampling phylloxera from another vineyard block. Join-count spatial autocorrelation statistics were used to explore fine-scale spatial structure. Clones were nonrandomly distributed within the block and there was evidence that the distribution of clones followed rows. These findings suggest firstly that there is limited dispersal of root and leaf feeding phylloxera, and secondly that factors, other than vine host, are likely to be important and contribute to clonal structure within populations.

Spatial Autocorrelation of Genetic Variation in Three Stands of Ophiopogon xylorrhizus(Liliaceaes.l. )

Using spatial autocorrelation analysis, the spatial distribution of genotypes and gene frequencies at three allozyme loci, as well as the spatial distribution of family outcrossing rates were investigated in three stands of the tropical rainforest endangered perennial Ophiopogon xylorrhizus in Southwest China. Similar spatial patterns of the different allozymes were found both in individual stands and among stands, particularly at the whole distance classes. There were slightly different associations between like-homozygous and like-heterozygous plants. Most like-homozygous plant pairs exhibited a significant positive autocorrelation in shorter distance classes, and a random distribution or slightly negative autocorrelation in the following one or two distance classes. In contrast, a significant positive autocorrelation was found in the last one or two distance classes, indicating that like-homozygous plants were clustered in patches with a diameter of 5–10m. Most like-heterozygous plant pairs exhibited a random distribution or a slightly negative autocorrelation in shorter distance classes, a significantly positive autocorrelation in the next one or two distance classes, and tended to a random distribution in the last one or two distance classes. Fifty-five per cent of unlike-homozygous plant pairs existed in different patches with similar patch sizes and these patches did not overlap. Highly consistent patterns were found in the spatial distribution of gene frequencies. Different genes existed in different patches, with a highly consistent average patch size in all three stands. Family outcrossing rates were randomly distributed in space. These results imply that it is not possible to sample the genetic variation of a population by merely conserving one part of it because the population is unlikely to be homogenous.

Genotypic structure in clonal Rhododendron ferrugineum L. (Ericaceae) populations: origin and maintenance

Genotypic structure of a closed population of the clonal ericaceous shrub Rhododendron ferrugineum is examined in the light of two independent studies previously conducted on this species. In the first study, spatial distribution of genotypes in the closed population was inferred from the amplified fragment length polymorphism (AFLP) procedure. Age of clones was estimated using their spatial extent and the annual growth rate of shoots. In the second study, ramet demography was studied in the three most representative stages of shrub invasion on two different sites, including the site where the population investigated by AFLP grew. The demographic data recorded were the area occupied by ramets and ramet age, and from this information the developmental pattern of Rhododendron populations was determined. Additional data such as genet density and distance between genets were calculated. These two sources of information allow us to propose that all or most of the clones detected in the closed population established at the early successional stage, and that the present genotypic structure was established several hundred years ago, long before the population reached total closure. Hypotheses concerning the future development of this genotypic structure are discussed.

SPATIAL GENETIC STRUCTURE IN POPULATIONS OF HOSTA CAPITATA AND HOSTA MINOR (LILIACEAE)

The spatial distribution of genotypes was examined in each of two populations ofHosta capitataandH. minorusing spatial autocorrelation analysis of enzyme polymorphisms. Although these two species have a similar life history and similar ecological traits, this study indicates that the pattern of spatial genetic distribution in the species differed. The percentage of significant Moran'sIfor all distance classes differs (20% and 14% inH. capitata;33% and 53% inH. minor).Significant positive Moran'sIvalues were detected in the shorter distance classes forH. capitata(distance class 1–3, 0–4.5 m), whereas these values forH. minorwere observed in longer distance classes (distance classes 5–6, 0–10.4 m). Approximate minimum patch area forH, capitata(7–20 m2) is smaller than that ofH. minor(38–78 m2). As the weights of seeds are nearly the same in both species and seedlings grow near maternal individuals in both species, and gene flow via seed is expected to be at the same level, the observed difference in patterns of spatial genetic distribution may in part be due to the differences between both species in terms of the extent of gene flow via pollen dispersal, selective forces, and other unknown factors of their reproductive biology.

Spatial autocorrelation of allozyme traits in a Norway spruce (Piceaabies) population

Using isoenzymes as gene markers and spatial autocorrelation analysis as a tool to detect spatial patterns, we studied the spatial distribution of genotypes in a naturally regenerated uneven-aged Norway spruce (Piceaabies (L.) Karst.) stand on the eastern Italian Alps. In most cases we found a random distribution of genotypes in space; in the whole data set less than 11% of genotype pairs showed positive associations for the first (10 m) distance class. Extensive gene flow, due to long distance dispersal of pollen and seeds in P. abies, may account for the observed spatial patterns. A few genotypes (GotB-22, LapB-23, LapB-24, SkdB-12, and MnrB-12) showed a significant clumped distribution over a small spatial scale. We suggested that selection processes driven by environmental variability might have produced significant clumping of these genotypes. However, the role of factors linked to the breeding system, and of chance events, in determining the population spatial structure cannot be excluded in our study.

The spatial genetic structure in natural populations of the Australian temperate rainforest tree Atherosperma moschatum (Labill.) (Monimiaceae)

The spatial distribution of genotypes at six enzyme loci was investigated in 17 natural populations of the temperate rainforest tree Atherosperma moschatum using spatial autocorrelation. The results from the different enzymes were consistent at each population as well as among populations, particularly at short distances. There were stronger associations overall between like homozygous trees than like heterozygous trees. Unlike trees were generally significantly negatively autocorrelated at short distances. Two consistent patterns of associations were found with increasing distances between trees. A highly consistent average patch length was found in most populations for the enzymes studied. The results suggest that there are small neighbourhood sizes in this species and therefore small stands are worth conserving. In addition, larger populations are unlikely to be homogenous. Therefore, conserving one part of a population may not adequately sample its genetic variability.

Patterns of Allozyme Variation in Three Costa Rican Species of Piper

Allozyme variation was studied in sympatric populations of three species of Piper (Piperaceae) in Parque Nacional Santa Rosa, northwestern Costa Rica. Levels of intrapopulational genetic diversity for all three species are low relative to other plants (proportion of polymorphic loci = 2/21 for P. amalago, 1/24 for P. jacquemontianum, and 0/20 for P. pseudo-fuligineum). Genotype frequencies at one polymorphic locus (b-Aatl in P. amalago) are significantly heterogeneous among subpopulations within the Santa Rosa population. Within subpopulations of P. amalago, heterozygote frequencies are consistent with the estimated effective outcrossing rate (0.58) at one locus (b-Aatl) but are in excess at the other locus (Pgi2). Nei's genetic distances between species are relatively high compared to other pairs of congeneric plants, ranging from 0.454 to 0.988. These high genetic distances are consistent with the idea that this large genus of flowering plants is relatively old. PATTERNS OF ALLOZYME-GENETIC VARIATION have been examined for a large number of plant species and have provided important insights into a number of evolutionary and taxonomic questions (Brown 1979, Hamrick et al. 1979, Gottlieb 1981, Loveless & Hamrick 1984). However, these studies are strongly biased toward herbaceous plants and temperate climates. Despite the immediate and practical need for increased understanding of the genetic structure and dynamics of tropical plant species (Frankel & Soule 1981, Schonewald-Cox et al. 1983), few such species have been assayed for levels and patterns of allozyme variation. In the present paper we present the results of a study of allozyme variation in three shrubby species of Piper (Piperaceae) which occur sympatrically within Parque Nacional Santa Rosa in northwestern Costa Rica. Our interest in this genus is motivated by two considerations. First, seed dispersal in Santa Rosa is mediated exdusively by frugivorous bats which may transport fruits relatively long distances (Heithaus & Fleming 1978). The generalization that gene flow is insufficient to prevent random genetic divergence of conspecific plant populations (Levin & Kerster 1974, Levin 1981) may not hold for such species (Loveless & Hamrick 1984), so it is of interest to ascertain whether spatial genetic structure is present within Santa Rosa. Second, Piper is one of the most speciose genera of flowering plants (Gentry 1982), suggesting it may be relatively old. The three species studied here are not considered to be closely related within the genus (Burger 1971), so it is of interest to determine whether the genetic distances among these species are consistent with a relatively ancient divergence. Thus we have addressed three specific questions in this study: (1) how much allozyme variation is present in these three species of Piper; (2) is there significant allozyme divergence between subpopulations within Santa Rosa; and (3) how do the genetic distances between these species compare with genetic distances between other congeners? MATERIALS AND METHODS Individuals of Piper amalago L., P. pseudo-fuligineum C.DC., and P. jacquemontianum Kunth were sampled from nine sites, or subpopulations, within Parque Nacional Santa Rosa, in northwestern Costa Rica during June and July of 1983 (Fig. 1, Table 1). Fleming (1985) has described the study site and natural history of these and two other sympatric species of Santa Rosa Pipers. Briefly, the three species investigated electrophoretically are common but patchily-distributed shrubs that require habitat disturbance to become established. Their flowers are visited by generalist, pollen-collecting insects, and exdosure experiments indicate that insect visitation is necessary for high seed set (Fleming, unpubl. data). Areas sampled ranged in size from 0.1 to 0.5 ha. Although connections between plants were not observed in the field, it is possible that some of the individuals sampled were ramets of the same done which are no longer connected. This would decrease the effective sample size per subpopulation and bias the sample toward larger dones. While this possibility exists, the electrophoretic results (below) reveal multiple genotypes per site for both P. amalago and P. jacquemontianum. The spatial distribution of genotypes among mapped individuals of P. amalago at several sites indicates that clones, if present, must be small relative to the area sampled (Heywood & Fleming, unpubl. data). Young leaves from established plants in the field served i Received 30 July 1984, revision accepted 23 April 1985. 208 BIOTROPICA 18(3): 208-213 1986 This content downloaded from 157.55.39.251 on Fri, 13 May 2016 05:20:25 UTC All use subject to http://about.jstor.org/terms

Homozygosity and patch structure in plant populations as a result of nearest-neighbor pollination

The population genetic consequences of nearest-neighbor pollination in an outcrossing plant species were investigated through computer simulations. The genetic system consisted of two alleles at a single locus in a self-incompatible plant that mates by random pollen transfer from a neighboring individual. Beginning with a random distribution of genotypes, restricted pollen and seed dispersal were applied each generation to 10,000 individuals spaced uniformly on a square grid. This restricted gene flow caused inbreeding, a rapid increase in homozygosity, and striking microgeographic differentiation of the populations. Patches of homozygotes bordered by heterozygotes formed quickly and persisted for many generations. Thus, high levels of inbreeding, homozygosity, and patchiness in the spatial distribution of genotypes are expected in plant populations with breeding systems based on nearest-neighbor pollination, and such observations require no explanation by natural selection or other deterministic forces.