Class Note for ECOL 485 at UA
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Effects of Competition Colonization and Extinction on Rodent Species Diversity Thomas J Valone James H Brown Science New Series Vol 267 No 5199 Feb 10 1995 880 883 Stable URL httplinksjstororgsicisici0036807528199502102933A2673A51993C8803AEOCCAE3E20CO3B2U Science is currently published by American Association for the Advancement of Science Your use of the J STOR archive indicates your acceptance of J STOR s Terms and Conditions of Use available at httpwwwjstororgabouttermshtm1 J STOR s Terms and Conditions of Use provides in part that unless you have obtained prior permission you may not download an entire issue of a journal or multiple copies of articles and you may use content in the J STOR archive only for your personal noncommercial use Please contact the publisher regarding any further use of this work Publisher contact information may be obtained at httpwwwj stororgjournalsaaashtml Each copy of any part of a J STOR transmission must contain the same copyright notice that appears on the screen or printed page of such transmission J STOR is an independent notforprofit organization dedicated to creating and preserving a digital archive of scholarly journals For more information regarding J STOR please contact support j stororg httpwwwj stororg Tue Aug 16 183906 2005 previous year s shrub shoots is not included in our estimates 21 Forforbs AGB LAI X 4994 i 476 gm2 N 10 and AGB percent of areal coverage X 279 gm2 N 10 For graminoids AGB LAI X 2620 i 234 gm2 N 10 and AGB percent of areal coverage X 219 t 023 gm2 N 10 For shrubs incremental AGB incremental shoot length in cen timeters per square meter X 0028 0005 gm2 N 19 and incremental shoot length percent of areal coverage X 1718 113 cmm2 N 10 Regressions of LAI or shoot length against the direct measure of AGB carried out separately for each zone and for each treatment showed no signi cant depen dence of regression coef cients on zone or treatment 22 F S Chapin Ill and G R Shaver Ecology 66 564 1985 23 M D Walker P J Webber E H Arnold D Ebert May ibid 75 393 1994 24 The need to consider the climate sensitivity of all life stages of vegetation is emphasized by F l Wood ward and B G Williams Vegetatio 69 189 1987 25 C Whitlock and P J Bartlein Quat Res NY 39 231 1993 26 M S Kearney and B H Luckman Science 221 261 1983 Although we did not find a Quaternary record specifically from the western Rocky Moun tains the evidence cited is from other regions with in the Cordillera 27 The importance of reliable forecasts of interannual climatic variability is underscored by our observa tion that the magnitude of the effect of heating on AGB varied with summer soil moisture levels see also 23 and N L Stephenson Am Nat 135 649 1990 28 Investigation of climate controls on growth of tundra species shows that within the broad grouping of forbs or shrubs species at a particular site can differ Effects of Competition Colonization and Extinction on Rodent Species Diversity Thomas J Valonef and James H Brown Analyses of longterm experimental data from the Chihuahuan desert revealed that spe cies diversity of other rodents was higher on plots from which kangaroo rats Dipodomys spp had been removed The difference was due to consistently higher colonization and lower extinction probabilities of small granivorous rodents in the absence of competitively dominant kangaroo rats The results of this ecosystem experiment demonstrate the importance of both competitive exclusion and metapopulation dynamics for biological diversity in a natural community Theoretical and laboratory studies suggest that the number of species that coexist in a community is in uenced by several process es including competition predation mutu alism disturbance and physical stress 1 6 Although there is evidence that each of these factors can affect the relative abun dance of species in a natural community there are few examples of how either singly or in combination they affect the number of coexisting species Further in order for these processes to alter species diversity rather than just relative abundance they must af fect metapopulation dynamics that is the probabilities of local colonization and ex tinction 6 8 Mathematical models predict and labo ratory experiments have demonstrated com petitive exclusion the capacity of a com petitor to cause the local extinction of an other species 1 9 Evidence for competi tive exclusion from field studies is more equivocal 4 10 13 Examples of extinc tion of species native to islands and other isolated habitats are difficult to explain sole ly by competition from invading exotics be cause such extinctions almost always coin Department of Biology University of New Mexico Albu querque NM 87131 USA Present address Biology Department California State University Northridge Northridge CA 91330 8303 USA tTo whom correspondence should be addressed 880 SCIENCE cide with increased human influence 14 Here we demonstrate the effect of com petition on species diversity in a communi ty of rodents in the Chihuahuan desert The removal of seedeating kangaroo rats Dip odomys spp from small experimental plots increased the number of small granivorous rodent species These increases in diversity resulted from higher probabilities of coloni zation or lower probabilities of extinction or both In 1977 experimental plots 50 by 50 m were established on a 20ha site of relatively homogeneous Chihuahuan desert scrub veg etation near Portal Arizona Each plot was surrounded by wire mesh topped with alumi num flashing to control access by rodents Plots were assigned various experimental treatments that included exclusion of differ ent rodent species on the basis of body size Equal access or control plots had 16 large 37 by 57 cm gates in the wire mesh which allowed access to all rodents Plots from which kangaroo rats were removed had 16 small 19 by 19 cm gates in the wire mesh which allowed access by all small bodied rodents but prevented the larger bod ied kangaroo rats Dipodomys spectabilis D merriami and D ordii from entering From 1977 to 1987 there were 14 equal access and 4 kangaroo rat removal plots In 1988 some equal access plots were converted into new kangaroo rat removal plots by changing their VOL 267 10 FEBRUARY 1995 markedly in their responses to environmental in u ences 22 29 We thank C Still and L Tucker for assistance with fieldwork and N Arens for her help in understanding the Quaternary pollen record We also thank M Loik M Price and N Waser for advice C D Antonio J Dunne S Hobbie A Kinzig M Loik S Saleska K Shen M Tom and M DeLapa for editorial com ments and the staff of RMBL for support This work was supported by grants from the National Science Foundation DEB 9207588 and BSR 9020579 the US Department of Agriculture s Cooperative State Research Service CA B SSC 51 13 H the Pew Charitable Trusts by a predoctoral fellowship from the US National Aeronautics and Space Adminis tration and by a John Simon Guggenheim Founda tion fellowship 25 May 1994 accepted 9 November 1994 gate size Thus from 1988 to the present March 1994 there were eight equal access plots four old kangaroo rat removal plots Dipodomys excluded continuously since 1977 and four new kangaroo rat removal plots Dipodomys excluded continuously since 1988 Approximately each month since 1977 rodents on all plots have been censused For one night 49 Sherman live traps were placed on each plot and baited with millet During each census all gates were closed so that only resident individuals were caught All individuals captured were identified mea sured uniquely marked and released 13 Kangaroo rats compete strongly with other rodents 15 and have large effects on vegetation 16 17 Within 1 to 2 years of removal of kangaroo rats from experimental plots densities of small seed eating rodents increased more than twofold 13 15 8 to 10 years after kangaroo rat removal annual and perennial grass density increased more than threefold 16 17 Species diversity of the other nocturnal rodents changed in response to experimen tal removal of kangaroo rats During the past 17 years a total of 14 such species have been recorded Kangaroo rat removal plots usually supported more species of other ro dents than did equal access plots Fig 1 From 1977 to 1981 there was no differ ence in the number of nonDipodomys spe cies caught per month between equal access and kangaroo rat removal plots Table 1 From 1982 to 1987 and again from 1988 to 1994 however there were more rodent spe cies per month on kangaroo rat removal plots than on equal access plots Table 1 The increasing trend in species diversity can be attributed in part to the indirect effect of kangaroo rats on rodents through vegetation 18 19 as well as to the direct effect of the relaxation of competition From 1982 to total number of species each month is small there is roughly a 60 difference in species richness between equal access and kangaroo rat removal plots During this study overall local diversity total number of species ob served averaged approximately 27 higher on kangaroo rat removal plots than on equal access plots Table 1 Compared to equal access plots kangaroo rat removal plots sup ported more individuals as well as more spe cies of other rodents For example during the period 1988 to 1994 an average of 1175 individuals of nonDipodomys species were captured on equal access plots versus an av erage of 231 individuals of nonDipodomys species captured on kangaroo rat removal plots Thus one possible explanation of our results is that kangaroo rat removal plots contained more species simply because they contained more individuals 18 Rarefac tion techniques 19 20 take account of how the number of individuals sampled af fects observed species richness and permit comparison of the number of species expect ed in samples of equal numbers of individu als Application of rarefaction revealed that when 60 individuals were captured the few est nonDipodomys individuals captured on any plot kangaroo rat removal plots con tained significantly more species 926 than did equal access plots 795 species P 001 MannWhitney U test Thus the in creased species richness on kangaroo rat re moval plots was not simply due to the greater numbers of individuals sampled Equal access plots typically contained one kangaroo rat species and after 1982 kangaroo rat removal plots contained about one more species of small granivore per month than did equal access plots Table 1 Colonization by small granivores there fore was sufficient to maintain total small mammal species diversity on the experi mental plots These results contrast with recent observations on plant communities that showed that colonization by new spe cies was not sufficient to compensate for local extinctions 21 Our data also provide insight into the mechanisms that generated the differences in species richness Evenness 20 was sig nificantly lower on kangaroo rat removal plots during 1988 to 1994 kangaroo rat removal I 079 equal access I 088 P 001 MannWhitney U test more rare species were able to coexist in the absence of kangaroo rats The occurrence of these rare species on kangaroo rat removal plots should be attrib utable to some combination of higher rates of colonization and lower rates of extinction No species showed a significantly higher col onization probability on equal access plots than on kangaroo rat removal plots during either 1982 to 1987 or 1988 to 1994 Table 2 22 From 1982 to 1987 only Reithrodon tomys megalotis had a significantly higher colonization probability on plots from which kangaroo rats had been excluded From 1988 to 1994 however five species exhibited sig nificantly higher colonization probabilities on kangaroo rat removal plots More impor tant four of the five species had similar colonization success on both old and new kangaroo rat removal plots indicating that colonization probability for these species was related to the absence of kangaroo rats and not to the differences in vegetation structure on these plots The remaining species Sig modon fulviventer which is folivorous and characteristic of grassland habitats 23 ex hibited a higher colonization probability on old kangaroo rat removal plots than on new kangaroo rat removal plots indicating that its colonization was influenced by the in creased grass cover on old kangaroo rat re moval plots No species had a significantly higher ex tinction probability on kangaroo rat removal plots compared with equal access plots Ta 39 ble 3 Peromyscus maniculatus had a signifi cantly lower extinction probability on kan garoo rat removal plots from 1982 to 1987 REPORTS whereas Perognathus aws and R megalotis had significantly lower extinction probabili ties on kangaroo rat removal plots from 1982 to 1994 From 1988 to 1994 Peromyscus eremicus had a significantly lower extinction probability on old kangaroo rat removal plots whereas Chaetodipus penicillatus had a signif icantly lower extinction probability only on new kangaroo rat removal plots compared with equal access plots Two mechanisms contributed to the higher species diversity of other rodents on kangaroo rat removal plots The most impor tant process was a relaxation of competitive exclusion Six of the eight granivorous ro dent species had higher colonization proba bilities or lower extinction probabilities or both Tables 2 and 3 None of the ro dents exhibited the opposite pattern which would be expected if there were strong indirect effects of competition among the rodents such that the removal of kangaroo rats would benefit a subset of small granivores which would then com petitively inhibit other species Kangaroo rats compete with seedeating rodents both by reducing their food supply 4 Equal access g New kangaroo rat removal 539 Old kangaroo rat removal r 5 3 f g Iquot quot1 5quot 5393 539 339 g 39quot i t 8 a r quot5 n 1 0 u39 f 39 39 U i 3 39g i n 3939 Hquot i 4 2 539 t r 393 5 l e 2 3 5 5 l39 39 19 E 1 39f 39g I in i 1 1 l l 1 s 1 quot l Iii o f r mhwu WFU un mrr 1978 1980 1982 1984 1986 1988 1990 1992 1994 Date Fig 1 Effect of different treatments on mean number of species per plot per month Data are plotted as 3 month averages for clarity in all analyses of data from 1988 to 1994 old versus new kangaroo rat removal plots were compared If no significant difference was detected the data from these plots were combined to compare kangaroo rat removal plots to equal access plots Table 1 Comparison of the number of non kangaroo rat species on treatment plots by month and over SCIENCE years Treatment 1 977 1 981 1 982 1 987 1 988 1994 Mean number of species per plot per month Equal access 083 128 1 20 Kangaroo rat removal 099 1 93 209 Mean total number of species observed Equal access 61 76 86 Kangaroo rat removal 80 93T 1 1 01 P lt 005 equal access versus kangaroo rat removal treatment repeated measures analysis of variance with plots as unit of replication and months as repeated measures removal treatment MannWhitney U test VOL 267 39 TPltO1 10 FEBRUARY 1995 iP lt 005 equal access versus kangaroo rat 881 and by aggressive defense of space 13 23 24 Nongranivorous rodent species charac teristic of desert shrub habitats folivorous Neotoma albigula and insectivorous Onych omys leucogaste39r and O torridus which pre sumably do not compete strongly with kan garoo rats did not respond to kangaroo rat removal Tables 2 and 3 Whereas the re sponses observed largely represent habitat selection by individual rodents all species have persisted on the plots for at least several months and been observed in reproductive condition 25 A second mechanism of lesser impor tance was the effect of kangaroo rats on vegetation The increase in grass cover due to the longterm removal of kangaroo rats resulted in significantly increased coloniza tion of one species characteristic of grassland habitats Table 2 Thus competitive exclu sion apparently accounted for six of the sev en species that contributed to the higher diversity on kangaroo rat removal plots 26 Our results show that interspecific com petition affects diversity of this desert ro dent community by altering metapopula tion dynamics Although early theoretical studies examined the effect of colonization Table 2 Mean colonization probabilities according to the method of Clark and Rosenzweig 22 1982 1987 1988 1994 Kangaroo rat Spec39es Equal Kangaroo Equal removal access rat removal access Old New Granivores Baiomys taylori 0 0 0004 0063 0011 Chaetodipus penicillatus 0040 0060 0076 0137 0191 C hispidus 0003 0012 0006 0004 0004 Perognathus avus 0045 0068 0039 0121 0129 Peromyscus eremicus 0106 0132 0093 0127 0144 Peromyscus maniculatus 0059 0220 0002 0020 0020 Reithrodontomys fulvescens 0 0008 0006 0019 0020 R megaotis 0094 0311 0152 0294 0285 Nongranivores Sigmodon fulviventer 0 0 0012 0050 00081 8 hispidus 0006 0012 0017 0060 01 S ochrognathus 0004 0018 0016 Neotoma albigula 0193 0159 0096 0063 0071 Onychomys Ieucogaster 01 76 01 44 01 07 0066 0095 O torridus 0154 0136 0161 0149 0137 P lt 010 P lt 005 P lt 001 P lt 0001 equal access versus kangaroo rat removal treatment t test TP lt 005 old versus new kangaroo rat removal plots t test Table 3 Mean extinction probabilities 22 1982 1 987 1 988 1 994 Specnes Equal Kangaroo Equal Kangaroo rat removal access rat removal access Old New Granivores Baiomys taylori 0999 0529 09991 Chaetodipus penicillatus 0531 0535 0562 0347 0312 C hispidus 0999 0393 0620 0500 0250 Perognathus avus 0576 0208 0686 0348 0263t Peromyscus eremicus 0569 0625 0636 0366 0378 Peromyscus maniculatus 0721 0468 0999 0734 0611 Reithrodontomys fulvescens 0999 0687 0827 0386 R megaotis 0608 0324 0548 0230 0283 Nongranivores Sigmodon fulviventer 0585 0843 0670 S hispidus 0813 0830 0485 0494 S ochrognathus 0999 0333 0770 Neotoma albigula 0389 0577 0999 0529 0525 Onychomys Ieucogaster 0662 0623 0636 0748 0758 O torridus 0663 0708 0518 0564 0488 P lt 01 P lt 005 P lt 001 P lt 0001 equal access versus kangaroo rat removal treatment t test TP lt 01 P lt 005 old versus new kangaroo rat removal plots t test 882 SCIENCE VOL 267 10 FEBRUARY 1995 and extinction on the coexistence of com peting species 27 most recent models of metapopulation dynamics have assumed that colonization and extinction are sto chastic processes 8 Our results suggest that competition can have deterministic ef fects on probabilities of local colonization and extinction Experimental studies have shown that the presence or absence of a top predator can propagate through a food web marked ly altering the diversity and composition of a community 2 28 Our results suggest that the presence or absence of a dominant competitor can have similar effects Addi tions or deletions of such species change the competitive environment and alter the col onization and extinction probabilities of other species Such cascading effects of competition have implications for conser vation policies because the extinction of native species establishment of exotics and reintroduction of extirpated species will of ten cause further changes in diversity REFERENCES AND NOTES 1 G J Cause The Struggle for Existence Williams and Wilkins Baltimore 1934 2 R T Paine Am Nat 100 65 1966 3 M L Cody and J M Diamond Eds Ecology and Evolution of Communities Harvard Univ Press Cam bridge MA 1975 J M Diamond and T J Case Eds Community Ecology Harper and Row New York 1986 J Kikkawa and D J Anderson Eds Community Ecology Blackwell Scienti c Melbourne Australia 1986 4 N G Hairston Sr Ecological Experiments Cam bridge Univ Press Cambridge 1989 5 S L Pimm The Balance of Nature Ecological Is sues in the Conservation of Species and Communi ties Univ of Chicago Press Chicago 1991 6 R E Ricklefs and D Schluter Eds Species Diver sity in Ecological Communities Univ of Chicago Press Chicago 1993 7 R Levins Bull Entomol Soc Am 15 237 1969 J H Brown and A KodricBrown Ecology 58 445 1977 l Hanski Oikos 38 210 1982 R H MacArthur and E O Wilson The Theory of Island Biogeography Princeton Univ Press Princeton NJ 1967 S L Pimm et al Am Nat 132 757 1988 8 M E Gilpin and l Hanski Eds Metapopulation Dy namics Empirical and Theoretical Investigations Cambridge Univ Press Cambridge 1991 9 T Park Ecol Monogr 18 265 1948 J H Vander meer Ecology 50 362 1969 R H MacArthur Geographical Ecology Harper and Row New York 1972 10 J H Connell Ecology 42 710 1961 11 P A Keddy Competition Chapman and Hall Lon don1989 12 M A Bowers and J H Brown Ecology 63 391 1981 13 J H Brown and J C Munger ibid 66 1545 1985 14 H A Mooney and J A Drake Eds Ecology of Biological Invasions of North America and Hawaii SpringerVerlag New York 1986 15 E J Heske et al Ecology 75 438 1994 16 J H Brown and E J Heske Science 250 1705 1990 y 17 E J Heske et al Oecologia 95 520 1993 18 F W Preston Ecology 43 185 1962 19 D S Simberloff in Ecological Diversity in Theory and Practice J F Grassle G P Patil W K Smith C Taillie Eds International Cooperative Publishing House Fairland MD 1979 pp 159 170 20 J A Ludwig and J F Reynolds Statistical Ecology Wiley New York 1988 21 D Tilman Ecology 74 2179 1993 22 C W Clark and M L Rosenzweig Am Nat 143 583 1994 For each period when a species was absent from a plot all subsequent time periods were searched to determine if a previously marked individ ual was recaptured on that plot If so the species was recorded as present on that plot for all periods be tween the two captures This prevented recording as apparent extinction and recolonization episodes indi viduals that were resident but not captured on a plot 23 D F Hoffmeister Mammals of Arizona Univ of Ari zona Press Tucson AZ 1986 24 R J Frye Oecoogia 59 74 1983 M A Bowers et a ibid 72 77 1987 M A Bowers and J H Brown ibid 92 242 1993 25 Maximum densities individuals per hectare calculat ed over 3month intervals for the most common spe cies are as follows D spectabiis 109 D merriami 173 D ordii 63 C penicilatus 52 P avus 31 P eremicus 50 P manicuatus 25 R megaotis 77 N albigua 31 O leucogaster 23 O torridus 27 See also J H Brown and Z Zeng Ecology 70 1507 1989 26 A third possible mechanism a difference in predation between kangaroo rat removal and equal access plots does not appear to be important All avian and mammalian predators had equivalent access to both types of plot The only predators differentially affected by our manipulations were large bodied snakes which were unable to pass through the small gates on kangaroo rat removal plots If such snakes had a significant effect however we would have expected Synergistic Roles for Receptor Occupancy and Aggregation in Integrin Transmembrane Function Shingo Miyamoto Steven K Akiyama Kenneth M Yamada Integrin receptors mediate cell adhesion signal transduction and cytoskeletal organi zation How a single transmembrane receptor can fulfill multiple functions was clarified by comparing roles of receptor occupancy and aggregation Integrin occupancy by monovalent ligand induced receptor redistribution but minimal tyrosine phosphorylation signaling or cytoskeletal protein redistribution Aggregation of integrins by noninhibitory monoclonal antibodies on beads induced intracellular accumulations of pp125FAK and tensin as well as phosphorylation but no accumulation of other cytoskeletal proteins such as talin Combining antibodymediated clustering with monovalent ligand occu pancy induced accumulation of seven cytoskeletal proteins including aactinin talin and Factin thereby mimicking multivalent interactions with fibronectin or polyvalent peptides lntegrins therefore mediate a complex repertoire of functions through the distinct effects of receptor aggregation receptor occupancy or both together Transmembrane integrin receptors for extracellular matrix proteins mediate sig nal transduction but they also participate in organizing cell adhesion sites and the actin containing cytoskeleton l How a single receptor can fulfill these multiple functions as well as show selectivity in effects on specific cytoskeletal proteins was examined by comparing the relative roles of receptor occupancy and aggrega tion We hypothesized that even though simple aggregation of a number of cyto kine and hormone receptors can mediate normal transmembrane signal transduc tion in place of ligand occupancy 2 some receptors might be able to assign distinct functions to three types of trans membrane signal ligand occupancy re ceptor aggregation or both together We determined the requirements for integrin mediated transmembrane control of the localization of specific cytoskeletal pro teins and for transmembrane signaling in volving tyrosine phosphorylation We found that i integrin receptors induce Laboratory of Developmental Biology National Institute of Dental Research National Institutes of Health Be thesda MD 20892 4370 USA To whom correspondence should be addressed SCIENCE 39 distinct cellular responses to binding of a ligand to aggregation or to a combination of the two ii integrin receptors can con trol the distribution of specific cytoskel etal proteins one protein was controlled by simple receptor aggregation whereas six other cytoskeletal proteins required a combination of both aggregation and li gand binding and iii the most proximal molecular interactions with integrin cyto plasmic domains appear to involve the cytoskeletal protein tensin and the ty rosine kinase pp125FAK focal adhesion kinase rather than the previous candi dates talin and oz actinin Polystyrene beads coated with substrates and antibodies served as mediators of inte grin occupancy or aggregation whereas sol uble monovalent ligands provided simple ligand occupancy Beads coated with fi bronectin mimic adhesive site generation 3 and can induce transmembrane aggre gation of a variety of cytoskeletal molecules including F actin Fig 1 A and B We developed procedures to quantitate this process and detected similar bead induced transmembrane aggregation with talin oz actinin tensin vinculin paxillin and filamin Fig 2A 4 5 In addition pp125FAK also showed bead induced clus VOL 267 10 FEBRUARY 1995 differences in colonization or extinction probabilities of Onychomys spp and N albigua which should be as susceptible to snake predation as granivorous ro dents Tables 2 and 3 see also 7 3 27 H S Horn and R H MacArthur Ecology 53 749 1972 M Slatkin ibid 55 128 1974 28 S L Pimm Food Webs Chapman and Hall Lon don 1982 S R Carpenter et a Bioscience 35 634 1985 S R Carpenter in Complex Interactions in Lake Communities S R Carpenter Ed Springer Verlag New York 1988 pp 119 135 29 We thank all those individuals who have contributed to the Portal Project overthe past 17 years Support ed by NSF most recently with grant DEB 9221238 5 July 1994 accepted 29 November 1994 tering whereas tubulin did not Figs 1C and 2A Similar results were obtained with beads coated with multivalent Arg Gly Asp RGD containing peptide conjugates Fig 2A and with adhesion blocking monoclonal antibodies mAbs to either the 0L5 or Bl subunit of the fibronectin receptor Fig 1 G to I and Fig 2B These multivalent antibody ligands induced trans membrane aggregation of all seven cy toskeletal proteins and FAK but not tubu lin lactate dehydrogenase or Jak Z Fig 1C 6 Little or no clustering of any cy toskeletal protein or FAK was observed with beads coated with polylysine or con canavalin A Fig 1 D to F and Fig 2A Unexpectedly mAbs that do not inhibit cell adhesion displayed a distinct and specif ic subset of these properties The mAbs 11 to the 0L5 subunit and KZO to the BI sub unit do not inhibit cell adhesion 6 9 and will hereafter be termed noninhibitory anti bodies Each of these noninhibitory mAbs coated on beads effectively induced trans membrane aggregation of tensin in patterns that were indistinguishable from tensin ag gregation induced by adhesion blocking mAbs and multivalent ligands Figs 1K and 2B However no significant coclustering of F actin talin oz actinin vinculin paxillin or filamin could be detected Figs 1 and 2B Interestingly FAK was also readily co clustered by these noninhibitory mAbs Figs 1L and 213 Three other mAbs to the BI subunit were compared and had similar pat terns for example noninhibitory mAbs DF5 and LM534 induced aggregation of FAK but not F actin whereas adhesion blocking mAb P4C10 induced aggregation of both FAK and F actin 6 It is now well established that ligand or antibody induced clustering of a number of growth factor hormone and other recep tors can mediate transmembrane signal transduction including integrin mediated tyrosine phosphorylation of FAK 2 10 As expected if simple aggregation alone triggers integrin signaling beads coated with noninhibitory mAbs that do not bind to an active site stimulated tyrosine phos phorylation that could be detected by anti 883
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