Presence Of Congeners Research Articles

Stronger interspecific sexual differences may be favored when females search for mates in the presence of congeners

Why are some species sexually dimorphic while other closely related species are not? While all females in genus Strauzia share a multiply-banded wing pattern typical of many other true fruit flies, males of four species have noticeably elongated wings with banding patterns “coalesced” into a continuous dark streak across much of the wing. We take an integrative phylogenetic approach to explore the evolution of this dimorphism and develop general hypotheses underlying the evolution of wing dimorphism in flies. We find that the origin of coalesced and other darkened male wing patterns correlate with the inferred origin of host plant sharing in Strauzia. While wing shape among non-host-sharing species tended to be conserved across the phylogeny, shapes of male wings for Strauzia species sharing the same host plant were more different from one another than expected under Brownian models of evolution and overall rates of wing shape change differed between non-host-sharing species and host-sharing species. A survey of North American Tephritidae finds just three other genera with specialist species that share host plants. Host-sharing species in these genera also have wing patterns unusual for each genus. Only genus Eutreta is like Strauzia in having the unusual wing patterns only in males, and of genera that have multiple species sharing hosts, only in Eutreta and Strauzia do males hold territories while females search for mates. We hypothesize that in species that share host plants, those where females actively search for males in the presence of congeners may be more likely to evolve sexually dimorphic wing patterns.

The distribution of monogeneans of the genus Ligophorus on gills of the soiuy mullet Planiliza haematocheilus

The distribution on the host gills of three monogenean species of Ligophorus Euzet et Suriano, 1977, parasitising soiuy mullet Planiliza haematocheilus (Temminck & Schlegel, 1845) in the Black and Azov seas was studied. Specimens of L. pilengas and L. llewellyni prefer to localize on the same gill sites, namely, I-III gill arches and their medial sectors. The third species, L. kaohsianghsieni , was distributed on the gills more crowdedly, with about 80% of its specimens being present on II gill arch, occurring on the medial and ventral sectors approximately equally. In general, most monogeneans were found on the largest in area and well-watered gill arches and sectors, as well as the anterior hemibranchies, however within these areas they preferred to attach to proximal half of filaments with less amount of water current. The presence of congeners on gills had no significant effect on the distribution of L. pilengas and L. llewellyni . Increasing infrapopulation number resulted in a more uniform distribution of monogeneans on gills; their proportion upsurges significantly on the faster growing anterior hemibranchs as fish size increased.

Chloroplast DNA variation in a hyperdiverse tropical tree community.

We investigate chloroplast DNA variation in a hyperdiverse community of tropical rainforest trees in French Guiana, focusing on patterns of intraspecific and interspecific variation. We test whether a species genetic diversity is higher when it has congeners in the community with which it can exchange genes and if shared haplotypes are more frequent in genetically diverse species, as expected in the presence of introgression.We sampled a total of 1,681 individual trees from 472 species corresponding to 198 genera and sequenced them at a noncoding chloroplast DNA fragment.Polymorphism was more frequent in species that have congeneric species in the study site than in those without congeners (30% vs. 12%). Moreover, more chloroplast haplotypes were shared with congeners in polymorphic species than in monomorphic ones (44% vs. 28%).Despite large heterogeneities caused by genus‐specific behaviors in patterns of hybridization, these results suggest that the higher polymorphism in the presence of congeners is caused by local introgression rather than by incomplete lineage sorting. Our findings suggest that introgression has the potential to drive intraspecific genetic diversity in species‐rich tropical forests.

European rabbits recognise conspecifics in their predators’ diets

Rabbits can successfully avoid their enemies by evaluating the risk of predation. They have various defensive strategies, such as morphological adaptations and behaviours patterns, which enable them to perceive their predators and thus reduce the risk of predation. It is well documented that rabbits recognise the scats of terrestrial predators and avoid those areas in which they are present. However, few studies show whether the prey species can recognise the presence of congeners in carnivores’ scats, which would allow them to identify their predators in a more efficient manner. We have carried out a comparative analysis of the use of space made by rabbits on plots on which a neutral odour (water) or the odours of the ferrets’ scats that had consumed either rabbit or another mammal (beef) were applied. Our results showed a lower number of rabbit pellets on those plots containing predator odours than on the control plots. During the first 6 days after applying the first odour, the number of rabbit pellets was lower on plots on which rabbit had been included in the diet when compared with scats obtained from a beef diet. However, no differences between the two experimental plots were recorded during the third visit (9 days after applying the first odour). Our results suggest that rabbits may be able to detect congeners in their predators’ scats, thus leading them to, in the short term, avoid areas in which their terrestrial predators’ diet is based on conspecifics, probably as the result of them perceiving a higher risk of predation.

Simple prebiotic synthesis of high diversity dynamic combinatorial polyester libraries

It is widely believed that the origin of life depended on environmentally driven complexification of abiotically produced organic compounds. Polymerization is one type of such complexification, and it may be important that many diverse polymer sequences be produced for the sake of selection. Not all compound classes are easily polymerized under the environmental conditions present on primitive planets, and it is possible that life’s origin was aided by other monomers besides those used in contemporary biochemistry. Here we show that alpha-hydroxy acids, which are plausibly abundant prebiotic monomers, can be oligomerized to generate vast, likely sequence-complete libraries, which are also stable for significant amounts of time. This occurs over a variety of reaction conditions (temperature, concentration, salinity, and presence of congeners) compatible with geochemical settings on the primitive Earth and other solar system environments. The high-sequence heterogeneity achievable with these compounds may be useful for scaffolding the origin of life.

Determinants of spread in an urban landscape by an introduced lizard

Urban landscapes are a mixture of built structures, human-altered vegetation, and remnant semi-natural areas. The spatial arrangement of abiotic and biotic conditions resulting from urbanization doubtless influences the establishment and spread of non-native species in a city. We investigated the effects of habitat structure, thermal microclimates, and species coexistence on the spread of a non-native lizard (Anolis cristatellus) in the Miami metropolitan area of South Florida (USA). We used transect surveys to estimate lizard occurrence and abundance on trees and to measure vegetation characteristics, and we assessed forest cover and impervious surface using GIS. We sampled lizard body temperatures, habitat use, and relative abundance at multiple sites. At least one of five Anolis species occupied 79 % of the 1035 trees surveyed in primarily residential areas, and non-native A. cristatellus occupied 25 % of trees. Presence and abundance of A. cristatellus were strongly associated with forest patches, dense vegetation, and high canopy cover, which produced cooler microclimates suitable for this species. Presence of A. cristatellus was negatively associated with the ecologically similar non-native A. sagrei, resulting in reduced abundance and a shift in perch use of A. cristatellus. The limited spread of A. cristatellus in Miami over 35 years is due to the patchy, low-density distribution of wooded habitat, which limits dispersal by diffusion. The presence of congeners may also limit spread. Open habitats—some parks, yards and roadsides—contain few if any A. cristatellus, and colonization of isolated forest habitat appears to depend on human-mediated dispersal.

Potential for Interspecific Competition Between Congeneric Longhorned Beetle Species (Coleoptera: Cerambycidae) in an Adventive Environment.

The cerambycid beetle, Phoracantha semipunctata F., was introduced into California in the mid-1980s and killed large numbers of Eucalyptus host trees. The populations of the borer declined to very low levels in the mid-1990s following the establishment of the congener, Phoracantha recurva Newman, and the intentional introduction of the egg parasitoid, Avetianella longoi Siscaro. The distributions of the beetles overlap in the Australian native range, but one species has replaced the other in the adventive range in California. One possible explanation is differential susceptibility to natural enemies introduced for biological control. An alternative explanation for the reduced abundance of P. semipunctata is asymmetric interspecific competition between the two species. To test this hypothesis, equal larval densities of each species were introduced into host logs. In all cases, more P. recurva adults emerged than P. semipunctata adults, but the presence of congeners did not have a different effect than the presence of an equal density of conspecific individuals. Neither the temporal order of introduction or bark thickness altered the outcome of potential competitive interactions. Consequently, it appears that the ecological replacement of one borer with another in the adventive environment in southern California may not be a result of bottom-up intraguild competitive interactions. The top-down effects of natural enemies on P. semipunctata have most likely led to its decline.

Correction to Polybromodiphenyl Ethers and Decabromodiphenyl Ethane in Aquatic Sediments from Southern and Eastern Arkansas, United States

South-central Arkansas (AR) is home to major manufacturing facilities for brominated flame retardant chemicals (BFRs) in the U.S. Unintended release during production may have caused accumulation of the BFRs in the local environment. In this work, sediment cores were collected from six water bodies in AR, including three located close to the BFR manufacturing facilities in El Dorado and Magnolia, to investigate past and recent deposition histories. A total of 49 polybromodiphenyl ethers (PBDEs) and decabromodiphenyl ethane (DBDPE) were detected, with concentrations as high as 57 000 and 2400 ng/g dry weight for decabromodiphenyl ether (BDE209) and DBDPE, respectively. Log−log regression of BDE209 and DBDPE surface concentrations versus distance to known BFR manufacturing facilities fit the Gaussian Plume Dispersion model, and showed that, if the distance is shortened by half, concentrations of BDE209 and DBDPE would increase by 5-fold. The spatial distribution and temporal trend of the contamination indicate that the manufacturing of PBDEs and DBDPE is the primary source for these compounds in the environment of southern Arkansas. Interestingly, the occurrence of debromination of PBDEs in the sediments of a previously used wastewater sludge retention pond in Magnolia is indicated by the presence of congeners that had not been detected in any commercial PBDE mixtures and by increased fractions of lower brominated congeners relative to higher brominated congeners. Two unknown brominated compounds were detected in the sediments, and identified as nonabromodiphenyl ethanes. ■ INTRODUCTION Decabromodiphenyl ether (BDE209) is the main component of the commercial polybrominated diphenyl ether (PBDE) product nicknamed “deca”, which has been marketed since the late 1970s. In the U.S., deca has been marketed under the trade names Saytex102E of Albemarle Corp. and DE-83R of Chemtura Corp. The aggregated U.S. national production volume of deca was in the range of 10−50 million pounds in 1986 and 1990, and 50−100 million pounds in 1994, 1998, 2002, and 2006, according to the U.S. EPA Inventory Update Reporting (IUR) data. Following the cessation of penta and octa production, Albemarle and Chemtura, as well as the largest U.S. importer, ICL Industrial Products, Inc., announced their commitments to phase out deca in the U.S. by December 31, 2012, and to end all its uses by the end of 2013. Decabromodiphenyl ethane (DBDPE) was introduced in the mid-1980s as a replacement for deca. It is marketed in the U.S. under the trade names Saytex8010 by Albemarle and Firemaster 2100 by Chemtura. Information on global consumption of DBDPE is not available. The U.S. EPA IUR record of DBDPE production volume is in the range of 10−50 million pounds for 2006. DBDPE has been detected in the air, tree bark, herring gulls eggs and sediments in the environment of North America. South-central Arkansas, with the largest natural bromine reserves in the country and the largest bromine production in the world, is home to major manufacturing facilities for BFRs in the U.S. Despite the strict measures in controlling and preventing discharges during the manufacturing process and voluntary actions to reduce emissions, unintended release of the products to the environment may not be avoidable. PBDEs Received: March 29, 2012 Revised: July 11, 2012 Accepted: July 13, 2012 Published: July 13, 2012 Article

Polybromodiphenyl Ethers and Decabromodiphenyl Ethane in Aquatic Sediments from Southern and Eastern Arkansas, United States

South-central Arkansas (AR) is home to major manufacturing facilities for brominated flame retardant chemicals (BFRs) in the U.S. Unintended release during production may have caused accumulation of the BFRs in the local environment. In this work, sediment cores were collected from six water bodies in AR, including three located close to the BFR manufacturing facilities in El Dorado and Magnolia, to investigate past and recent deposition histories. A total of 49 polybromodiphenyl ethers (PBDEs) and decabromodiphenyl ethane (DBDPE) were detected, with concentrations as high as 57000 and 2400 ng/g dry weight for decabromodiphenyl ether (BDE209) and DBDPE, respectively. Log-log regression of BDE209 and DBDPE surface concentrations versus distance to known BFR manufacturing facilities fit the Gaussian Plume Dispersion model, and showed that, if the distance is shortened by half, concentrations of BDE209 and DBDPE would increase by 5-fold. The spatial distribution and temporal trend of the contamination indicate that the manufacturing of PBDEs and DBDPE is the primary source for these compounds in the environment of southern Arkansas. Interestingly, the occurrence of debromination of PBDEs in the sediments of a previously used wastewater sludge retention pond in Magnolia is indicated by the presence of congeners that had not been detected in any commercial PBDE mixtures and by increased fractions of lower brominated congeners relative to higher brominated congeners. Two unknown brominated compounds were detected in the sediments, and identified as nonabromodiphenyl ethanes.

Establishment of introduced reptiles increases with the presence and richness of native congeners

Darwin proposed two contradictory hypotheses to explain the influence of congeners on the outcomes of invasion: the naturalization hypothesis, which predicts a negative relationship between the presence of congeners and invasion success, and the pre-adaptation hypothesis, which predicts a positive relationship between the presence of congeners and invasion success. Studies testing these hypotheses have shown mixed support. We tested these hypotheses using the establishment success of non-native reptiles and congener presence/absence and richness across the globe. Our results demonstrated support for the pre-adaptation hypothesis. We found that globally, both on islands and continents, establishment success was higher in the presence than in the absence of congeners and that establishment success increased with increasing congener richness. At the life form level, establishment success was higher for lizards, marginally higher for snakes, and not different for turtles in the presence of congeners; data were insufficient to test the hypotheses for crocodiles. There was no relationship between establishment success and congener richness for any life form. We suggest that we found support for the pre-adaptation hypothesis because, at the scale of our analysis, native congeners represent environmental conditions appropriate for the species rather than competition for niche space. Our results imply that areas to target for early detection of non-native reptiles are those that host closely related species.

Breeding habitat selection of sympatric White-tailed, Rock and Willow Ptarmigan in the southern Yukon Territory, Canada

We examined breeding habitat selection at two scales for White-tailed (Lagopus leucura), Rock (L. muta), and Willow Ptarmigan (L. lagopus) at an alpine site in the Ruby Range Mountains of the Yukon Territory, Canada. To infer species-specific preferences, we used logistic regres- sion and AIC model selection to compare nest habitat of White-tailed (n = 43) and Rock Ptarmigan (n = 58). Only descriptive statistics were used for Willow Ptarmigan (n = 8 nests) because of smaller sample sizes. Modeling results indicate elevation, slope, and the ground cover of graminoids and lichens were the main variables separating White-tailed and Rock Ptarmigan breeding habitat. Average estimates for elevation and slope around nest sites of each species followed a pattern of White-tailed Ptarmigan (1,827 m, 30) ( Rock Ptarmigan (1,728 m, 18) ( Wil- low Ptarmigan (1,512 m, 6). White-tailed Ptarmigan tended to select drier habitats dominated by lichens, rock and dwarf shrubs, while Rock Ptarmigan were more com- mon in meadows with graminoids and scattered woody shrubs. Willow Ptarmigan nested in areas with dense, woody shrubs. The three species display inter- and intraspecific territoriality, and while species-specific habitat preferences allow co-existence, the niche width of each is likely nar- rowed by the presence of congeners, particularly for White- tailed and Rock Ptarmigan. Within territories, all three species selected nest sites with more lateral cover than expected at random (24-50%). White-tailed and Rock Ptarmigan did not select specific cover types (vegetation or rock) for nest sites within territories, while most Willow Ptarmigan nests were located in patches of scrub birch (Betula glandulosa).

How strongly do interactions with closely‐related native species influence plant invasions? Darwin's naturalization hypothesis assessed on Mediterranean islands

AbstractAim Recent works have found the presence of native congeners to have a small effect on the naturalization rates of introduced plants, some suggesting a negative interaction (as proposed by Charles Darwin in The Origin of Species), and others a positive association. We assessed this question for a new biogeographic region, and discuss some of the problems associated with data base analyses of this type.Location Islands of the Mediterranean basin.Methods Presence or absence of congeners was assessed for all naturalized alien plants species at regional, local and habitat scales. Using general linear models, we attempted to explain the abundance of the species (as measured by the number of islands where recorded) from their congeneric status, and assessed whether the patterns could be alternatively accounted for by a range of biological, geographical and anthropogenic factors. A simulation model was also used to investigate the impact of a simple bias on a comparable but hypothetical data set.Results Data base analyses addressing Darwin's hypothesis are prone to bias from a number of sources. Interaction between invaders and congenerics may be overestimated, as they often do not co‐occur in the same habitats. Furthermore, intercorrelations between naturalization success and associated factors such as introduction frequency, which are also not independent from relatedness with the native flora, may generate an apparent influence of congenerics without implying a biological interaction. We detected no true influence from related natives on the successful establishment of alien species of the Mediterranean. Rarely‐introduced species tended to fare better in the presence of congeners, but it appears that this effect was generated because species introduced accidentally into highly invasible agricultural and ruderal habitats have many relatives in the region, due to common evolutionary origins.Main conclusions Relatedness to the native flora has no more than a marginal influence on the invasion success of alien plants in the Mediterranean, although apparent trends can easily be generated through artefacts of the data base.

Predictors of Introduction Success in the South Florida Avifauna

Biological invasions are an increasing global challenge, for which single-species studies and analyses focused on testing single hypotheses of causation in isolation are unlikely to provide much additional insight. Species interact with other species to create communities, which derive from species interactions and from the interactions of species with the scale specific elements of the landscape that provide suitable habitat and exploitable resources. I used logistic regression analysis to sort among potential intrinsic, community and landscape variables that theoretically influence introduction success. I utilized the avian fauna of the Everglades of South Florida, and the variables body mass, distance to nearest neighbor (in terms of body mass), year of introduction, presence of congeners, guild membership, continent of origin, distribution in a body mass aggregation or gap, and distance to body-mass aggregation edge (in terms of body mass). Two variables were significant predictors of introduction success. Introduced avian species whose body mass placed them nearer to a body-mass aggregation edge and further from their neighbor were more likely to become successfully established. This suggests that community interactions, and community level phenomena, may be better understood by explicitly incorporating scale.

Population variation of web-invading spiders (Theridiidae: Argyrodes spp.) on host webs in Guam, Mariana Islands, Micronesia

Most Argyrodes spiders live in the webs of other spiders as kleptoparasites, stealing food from the host and scavenging small prey from the web. We observed three species of Argyrodes from the Micronesian island of Guam to learn (1) if Argyrodes species differ in their use of host-species webs, (2) whether some Argyrodes species occur more often in certain habitats, (3) whether population size of Argyrodes is a function of web size, and (4) how the presence of congeners affects population size on a web. Argyrodes lived most often in the large, long-lasting orb webs of Argiope appensa and Cyrtophora mollucensis, and, rarely, in the smaller, less durable orb webs of Neoscona spp. and Leucauge spp. Argyrodes argentatus and Argyrodes sp. A frequently co-occurred in Argiope webs, but A. argentatus was more common in beach strand and open disturbed habitats, while Argyrodes sp. A was more common in shaded native forest. The abundances of A. argentatus and Argyrodes sp. A in Argiope webs and the abundance of Argyrodes sp. B in Cyrtophora webs were positively correlated with the areas of the webs' prey-catching surfaces. The abundance of A. argentatus was inversely proportional to that of Argyrodes sp. A on Argiope webs and to that of Argyrodes sp. B on Cyrtophora webs, suggesting interspecific avoidance.

Spectrophotometric determination of phenol in the presence of congeners by multivariated calibration

The generation of poly-hydroxilated transient species during the photochemical treatment of phenol usually impedes the spectrophotmetric monitoring of its degradation process. Frequently, the appearance of compounds such as pyrocatechol, hydroquinone and benzoquinone produces serious spectral interference, which hinder the use of the classical univariate calibration process. In this work, the use of multivariate calibration is proposed to permit the spectrophotometric determination of phenol in the presence of these intermediates. Using 20 synthetic mixtures containing phenol and the interferents, a calibration model was developed by using a partial least square regression process (PLSR) and processing the absorbance signal between 180 and 300 nm. The model was validated by using 3 synthetic mixtures. In this operation, typical errors lower than 3% were observed. Close correlation between the results obtained by liquid chromatography and the proposed method was also observed.

Den choice and occupation patterns of shelters by two sympatric lobster species, Panulirus argus and Panulirus guttatus, under experimental conditions

Panulirus argus and P. guttatus are sympatric and commonly share dens. We studied their den-related behaviour in 3 m-diameter tanks, each holding a casita (low den) and a cave (high den) (1) separately (2) together (3) together with a triggerfish predator (Balistes vetula), and (4) separately with the predator. Most P. argus left shelter at night, and some during the day, and showed no den preference in any treatment. In caves, individual P. argus occupied all parts of caves in treatments 1 and 2 and mostly the vault (walls and ceiling) in treatments 3 and 4. Not all P. guttatus left shelters at night, and none during the day. Individual P. guttatus showed no den preference in treatments 1 and 2, preferred the cave in 3 and 4, and always preferred the cave vault. Presence of congeners did not affect den choice or cave position for either species, whereas the predator affected den choice (P. guttatus only) and position in caves (both species). Videotapes showed aggression between individual P. guttatus but not between individual P. argus or between species. The fish cohabited with lobsters in the casita but attacked them in caves and in the open. Unlike P. argus, P. guttatus exhibited no communal defence.

The Enigma of Mud Snail Shell Growth: Asymmetrical Competition or Character Displacement?

The growth rate and survival of Hydrobia rentrosa and H. ulvae from naturally co-existing populations were investigated when kept together and separately in cages settled in an intertidal pool. Population density ranged from 2800 to 22000 ind./m 2 . Density dependent mortality was not found. No influence of related species on mortality was observed. However, growth variation of snails suggested strong interspecific competition. Despite large differences in size of H. ulnae and young H. ventrosa used in the experiments (initial size ratio 1.8 ; final 1.4), the effect of interspecific competition was significant. Moreover, interspecific competition was asymmetrical. At population densities of 7000 14000 ind./m 2 with H. ulvae present, H. ventrosa grew slower than in single-species populations. In contrast, H. ulvae under the same conditions increased growth rate in comparison to single-species populations. The competition coefficient (α), as a function of population density based on growth rate individuals, was calculated. The proportion of inter- to intraspecific competition in their joint influence on Hydrobia growth rate depends on population density. This correlation was non-linear. With population density close to naturals H. ulvae growth is limited by intraspecific competition. H. ventrosa growth is limited by interspecific competition. It may be supposed that stable coexistence of these two species is possible only in populations dominated by H. ventrosa. Moreover, the larger the proportion of H. ventrosa, the higher should be the growth rate. In contrast, the correlation between growth rate of H. ulvae and proportion of this species should be negative. The population density and presence of congeners have a great impact on shell size as can be shown after only three months of cage experiments. The growth rate of the mud snails varies within the wide range of the norm response on food availability. This contradicts the opinion of the hereditary nature of character displacement of H. ulvae and H. ventrosa when they co-exist and allows the adoption of an environment-based explanation of the phenomenon.

INTEGRATIVE APPROACHES TO EVOLUTIONARY ECOLOGY: Anolis Lizards as Model Systems

Two approaches characterize the study of evolutionary ecology. Prospective studies investigate how present-day ecological processes may lead to evolutionary change; retrospective studies ask how present-day ecological conditions can be understood as the outcome of historical events. I argue that the most appropriate test of an evolutionary ecological hypothesis requires an integration of these approaches. I illustrate this approach by examining the hypothesis that interspecific competition has been the driving force behind the evolutionary radiation of Anolis lizards in the Caribbean. This hypothesis is supported by four lines of evidence: 1. Anole communities are structured by competition; 2. Populations alter resource use in the presence of congeners; 3. Microevolutionary adaptation occurs in response to resource shifts; and 4. Macroevolutionary patterns are consistent with interspecific competition as the driving force behind anole adaptive radiation.

Foraging habitat selection by American and Magellanic oystercatchers (Haematopus palliatus and H. leucopodus) on Patagonian tidal flats

I examined the foraging behaviour of American and Magellanic oystercatchers on Patagonian tidal flats in the context of several abiotic and social factors. I analyzed the simultaneous effect of tidal state, wind speed, temperature, habitat preference, and species abundance of one species on the other by means of multiway contingency tables and loglinear models. Diets of both species were similar and reflected the natural abundances of prey items in the tidal flats. Both species were strongly influenced by tidal state when selecting foraging habitat, but other factors modified their preferences. I found that the foraging behavior of American Oystercatchers was determined solely by abiotic factors; i.e., tidal state, foraging period, and a higher order interaction effect between wind speed and tidal state. By contrast, no combination of abiotic factors was sufficient to explain the observed abundances of Magellanic Oystercatchers. Instead, their foraging behavior was facilitated by the presence of congeners in addition to abiotic factors.

Specific alterations in the biological activities of C-20'-modified vinblastine congeners

Both the anti-tumor and toxic activities of the vinca alkaloid dimers, vinblastine (VBL) and vincristine (VCR), may reside at the level of their known cellular target, the microtubule system. The contributions made by each of the various actions of these alkaloids on this system are unknown. We have used new, complete synthetic methodologies to create a series of eight C-20' alkyl congeners of VBL and have examined these compounds for their abilities to (1) inhibit microtubule assembly, (2) disassemble preformed microtubules, and (3) induce spiral aggregate formation, using purified brain microtubule protein. By combining turbidimetric and electron microscopic techniques, we discovered that each of the various effects of VBL on the microtubule system in vitro was amenable to alteration by specific modification at this single molecular site. In addition, we report two new aberrations of VBL action—the induction of spirals by a concentration of congener below 1 μM and the formation of “opened” microtubules polymerized in the presence of congener. The relationship between anti-microtubule action in vitro and the cellular activities of growth inhibition and mitotic arrest by the congeners was examined in leukemic and colon cancer cell lines. In general, we found that both cellular perturbations were correlated to the ability of the congeners to inhibit microtubule polymerization rather than to the actions of spiral formation or microtubule disassembly. These results are a breakthrough in the structure/function relationship of the vinca alkaloid dimers and should provide the means to determine the role of specific anti-microtubule activities to the complex biological actions of these natural product drugs.