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Genetic Variation and Speciation Pj New World Cichlids

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Genetic Variation and Speciation Pj New World Cichlids GENETIC VARIATION AND SPECIATION PJ NEW WORLD CICHLIDS A thesis presented by Irving L. Kornfield to The Graduate School in partial fulfillment or the requirements for the degree of Master of Arts in Ecolozy and Evolution State University of New York at Stony Brook October, 1972 STATE UNIVERSITY OF NEW YORK AT STONY BROOK THE GRADUATE SCHOOL IR7I1G L. KO=FIELD We, the dissertation committee for the above candidate for the I.A. degree, hereby recommend acceptance of the dissertation. Richard K. Koehn (Dept. of Ecology and Evolution) Douglas J. Futuyma (Dept. of Ecology and Evolution) George C. Williwds (Dept. of Ecology and Evolution) The dissertation is accepted by the Graduate School. Dated : 1 Herbert deisenger, Dean Graduate School TABLE OF CONTENTS PAGE Abstract .................... Acknowledgments ................................... List of Figures ................................... List of Tables .................................... Introduction ...................................... Material and Methods .............................. Results ............................................ Discussion ........................................ Appendix ........................................... Bibliography ...................................... ABSTRACT Genetic variability was sCudieC rineteen electro- phoretic loci in two undescribed er-lc ,Achlids from Cuatro Cienegas, Coahuila, Mexico er(2. in Cichlosoma staTIg21 211. from the Rio Grande in Texas. The two endemic species differ markedly in their pharyngeal dentition and general ecologies. Only the Lactate Dehydro- genase was polymorphic in 130 endemic cichlids sampled from several lagunas. C. cyanogutt m was monomorphic at all loci examined in a sample of 38 individuals from the Rio Grande. The low proportion of polymorphic loci (=.055) and low average number of heterozygous loci per individual (=.003) differ markedly from all previous estimates. The data are not consistent with inbreeding, the founder effect, or low environmental variability as explanations for low genetic variability. No difference in isoenzyme mobility was observed between the two endemic cichlids (Rogers coefficient of genetic similarity, S = 1.0). C. cyanoguttatum exhibited identical mobility with the Cuatro Cienegas cichlids at more than half the loci examined (S = 0.582). The signifi- cant evolutionary divergence • that has occurred in certain morphological features of the endemic Cuatro Cienegas species. suggests that variability and similarity estimates based upon isoenzyme loci do not validly reflect genetic events associated with speciation. ACKNOWLEDGMENTS I would like to thank Mr. Gary L. Mr. K. Thompson, and Dr. Charles Waltace for the collec- tion of Rio Grande cichlids essential to this study, and Mr. Richard Stone for assistance during the collection of endemic cichlids in Mexico. A great debt is owed to Sr. Pepe Lugo, resident naturalist and entrepreneur, who cont'nues to provide invaluable assistance to all biological investigations in the Cuatro Cienegas Basin. Gratitude is expressed to Mr. John Kishpaugh, Dr. W. L. Minckley, Dr. Dennis Powers, and Dr. F. James Rohlf for assistance and advice in helping to solve many problems generated during the progress of this study. Appreciation is extended to Dr. George C. Williams for many interesting discussions and helpful suggestions. Dr. Douglas J. Futuyma's interest in the progress of this study was only surpassed by his numerous perceptive observations. I am grateful for his continued concern. Dr. Richard K. Koehn has, from the beginning, provided the ideas, tools, patience and enthusiasm necessary for the successful completion of this study and for my personal development. His uncompromising integrity and genuine interest has made it a pleasure to be his student. I express my deep appreciation to my wonderful wife, Tori, for her patience and understanding throughout the course of this study. This work was supported in part by research grants NSF (GB-25343) and USPHS (GM-28963) to Dr. Koehn and a ( SUNY Faculty/ Research Fellowship to Dr. Futuyma. 0-VAA, S OA: C,- US9H S Co-y -e-e v LIST OF FIGURES FIGURE PAGE Sample localities of C. cyanoguttatum at Mission, Texas, and achlosoma sm. from Cuatro Cienegas, Mexico 2 Sampling localities within the Cuatro Cienegas Basin 3 External morphology and pharyngeal bones of Cichlosoma spp ..................... 4 Zymograms of monomorphic isoenzymes Zymograms of variable isoenzymes ..... 6 Serum protein electroplerograms 11 A LIST OF TABLES TABLE PAGE Electrophoretic and staining methods .... Estimated genetic variability in the Cuatro Cienegas cichlids ....................... INTRODUCTION , Numerous studies on a wide dive- or organisms have appeared characterizing many isoenzymes as polymorphic. Many reports have inferred Atirly high levels of overall genomic variability in natural populations (e.g. Lewontin and Hubby, 1966: Selander et al, 1970; Ayala et al, 1972). These findings provide a basis from which to investigate major problems of evolutionary biology, notably the rela- tionship of variability to immediate and long-term survival of a species. Since most studied enzyme polymorphisms cannot be maintained by mutation alone (Tobari and Kojima, 1972), a variety of genetic mechanisms have been proposed to explain the continued existence of this variation. Selective neutrality of isoenzymes has repeatedly been suggested on theoretical grounds (Kimura, 1968; King and Jukes, 1969; King, 1972). There is presently, however, an overwhelming body of evidence implicating the role of natural selection in the maintenance of isoenzyme variability. While the selective agents responsible, or the selective mechanisms by which these may operate are not generally known (see tulmer, 1971; Koehn and Mitton, 1972) both population studies and biochemical observations have empirically demonstrated selective maintenance of enzyme variation (Koehn 1969; Merritt, 1972). Given different adaptations of alleles, environmental heterogeneity (or some component thereof) can act as a factor in the maintenance of genetic variability. (Levene, 1953; Levins and MacArthur, 1966; Prout, 19 ; Levins, 1968). From this theoretical basis, Koehn and Mitton (1972) demonstrated identical patterns of isoenzyme variation in two sympatric marine bivalves along an environmental gradient, suggesting an optimal response of different organisms to environmental heterogeneity. Powell (1971), in a laboratory study of Drosophilia willistoni over fifteen generations, demonstrated higher average heterozygosity and number of alleles per locus in populations maintained in heterogeneous environments. Variation in the environment of an organism is (particular- ly ia the aLscuLv of humeubLaLit. muchunisms) apparently reflected by the presence and continued existence of • beneficial adaptations responding to those varying environ- mental parameters. If an environmental signal is invariant over time (e.g. constant temperature), an allele with a selective advantage will eventually become fixed. The • contrasting case, however, is more complex. That is, if the environmental signal is not constant, the genetic strategy adopted at a responding locus could be fixation or polymorphism, depending upon the relative duration of alternate states of the signal (see Levins, 1968, for discussion). Electrophoretic comparisons of proteins between species (e.g. Hubby and Throckmorto 4)5, 1968; Selander et al, 1969; Johnston and Selander, 1971; Rockwood et al, 1971; Smith trid Koehn, 1971; Turner, 1972; Weber et al, 1972) have proviged insights into phyletic relationships and genetic reorganization during speciation. Although electrophoretic compari- sons have been successfully employed at the genus level, the systematic utility beyond congeneric comparisons is limited by an increasing probability of non-genetic identity (i.e., equivalent net charge but different amino acid composition). The extent of genomic reorgani- zation following speciation suggested by Hubby and ;, 4,‘ 1...a.rInen■uoj Jufwaypii-t-ka, tutu a cidnUer el al (1969) in Mus, remains controversial. Critical questions concerning genetic events during speciation remain to be answered. Some freshwater fish taxa are characterized by an ususually rapid proliferation of specialized species. "Explosive speciation" characterizes hundreds of cichlid species endemic to the Great Rift Valley lakes of Africa. Although a traditional view of the mechanics of their speciation is generally held (see Mayr, 1963), the complex reproductive habits (Baerends and Baerends-Van Roon, 1950) and rapidity of speciation within limited geographic areas (Greenwood, 1965) present an extremely unusual biological / situation that suggests numerous unexamined questions of evolutionary significance (see Greenwood, 1956; 1959; 1964; Fryer and Iles, 1969). Extreme modification of dentition and cranial morphology is commonly observed among the various trophic tAes (Greenwood, 1959; 1964). Additionally these fishes exhibit a high degree of intra- specific variation in just "...those characters of the animal which were utilized as a basis for specific differentiation by systematists...." (Fryer, 1959). This • study deals with Cichlasoma cyaglosuttatum, (Baird and Girard) which occurs throughout the major rivers of Texas and northern Mexico (Blair et al, 1957; Anonymous, 1967), • and three endemic undescribed species of Cichlosoma restricted to waters of the Cuatro Cienegas Basin, Coahuila, Mexico (Taylor and Minckley, 1966). While river environments
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