Spatial and Temporal Differentiation in Colonizing Populations of Ceratitis Capitata
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Heredity69 (1992)101—ill Received 2 August 1991 Genetical Society of Great Britain Spatial and temporal differentiation in colonizing populations of Ceratitis capitata ANNA R. MALACRIDA, CARMELA R. GUGLIELMINO*, GIULIANO GASPERI, LORELLA BARUFFI & RICCARDO MILANI Dipartimento di Biologia Animale, Universit di Pa via, Piazza Botta, 9, 27100 Pa via, and *D,partjmento di Genetica e Microbiologia, Universitá di Pa via ed Fstituto di Genetica e Biologia Evoluzionistica del CNR, Pa via, Italy Twoancestral populations (Kenya and Reunion), two Mediterranean (Procida and Sardinia) and one new American population (Guatemala) of Ceratitis capitata were examined by electrophoresis for genetic variability at 27 enzyme loci. Two ordination approaches (principal component analysis and a tree representation) and F-statistical analysis have been used to distinguish the various patterns of genetic variations and to infer the underline causes and their relative contribution to the total variation. Three main patterns of variation emerge from the data: geographical, annual and seasonal differentiation. A main part of intraspecific variability involves the differentiation of central (Kenya and Reunion) versus peripheral populations (the Meditterranean and the American populations). The analysis suggests that the genetic structure of these populations is correlated with the historical events of their colonization. The affinity of the Guatemalan population with the Kenyan one could be the result of a recent founding of this population from the source area (Africa). More ancient historical events of colonization characterize the two Mediterranean popula- tions. Seasonal variation has been found in the Procida population and chiefly involves the Mpi locus. In the same population the genetic variation across years has a minimum in 1986 due to the release of sterile T- 101 males. Keywords:Ceratitiscapitata, electrophoretic markers, genetic variability, geographical populations, seasonal variation, spatial and temporal differentiation. first record of its presence in the Americas in 1905 in Introduction Brasil. TheMediterranean fruit fly, Ceratitis capitata, is a poly- With respect to colonization, we suggest that the phagous and multivoltine tropical species which in the world medfly populations can be divided into three last 100 years has spread from its supposed origin in main categories. (i) ancestral populations: from the Africa to a number of countries including the Medi- African mainland, south of the Sahara, (ii) ancient terranean basin, parts of South and Central America populations: the Mediterranean populations; and (iii) and Australia (Fletcher, 1989a). Historically some of new populations: the American populations. Informa- the spread of the medfly has been documented (Hagen tion on dispersal and on colonization rate of this et al., 1981). As this species seems to have travelled species (Fletcher, 1 989b), coupled with knowledge of with man, it has been suggested that from its supposed the genetic structure of natural populations (Milani et origin (Southern Africa) it travelled from the east coast al., 1989), is crucial to gain an understanding of the of Africa to the Mediterranean with the Arab trade life-history strategies which determine the rate of gene (Maddison & Bartlett, 1989). In the Mediterranean flow between different sub-populations, and in plan- region the medfly exploits a great part of its biological fling management strategies against the pest species. potential because these areas offer an increasing Despite its economic importance, the species is still number of host fruits owing to the introduction of new poorly studied from the standpoint of population species of cultivars (Fimiani, 1989). The first record of genetics. Intraspecificgeneticdifferentiationis this pest in the European Mediterranean area was in expected to be associated with the colonization history Spain in 1842 (De Breme, 1842) and in Italy (Calabria) of C. capitata, despite the apparent morphological in 1863 (Martelli, 1910). Gallo etal. (1970) reports the uniformity within this species (White, 1989). Previous 102 A. R. MALACRIDA ETAL. studies have shown large genetic differences between host. Procida island was chosen as an experimental introduced populations and African ones (Huettel et area to study the field performance of sterilized males a!., 1980; Gasperi et aL,1987;Gasperi et a!., 1991). from genetic sexing strains (Robinson, 1989) in a Such data have provided information on the relative cooperative programme between ENEA (Italian contributions of selection, gene flow, genetic drift and National Conmiittee for Research and Development of geographical isolation on the genetic pattern of this Nuclear Energy) and IAEA (International Atomic species. Energy Agency) for the control of medfly using the SIT In this paper we examine two ancestral populations method (Sterile Insect Technique) (Cirio eta!., 1987). A (Kenya and Reunion), two ancient populations from total of 20.2 million sterile males from the strain T- 101 the Mediterranean (Procida and Sardinia) and one new (Seibersdorf, IAEA Laboratory) were released on the population: Guatemala. The aim is to describe the island of Procida from April 14 to July 13, 1986. The pattern of geographical and temporal variation and to Procida population was sampled over seasons, and infer the causes determining the relative similarity of consequently over density phases, from 1983 to 1986. the medfly populations. Mediterranean populations are A total of 22 samples comprising 751 specimens was interesting because winter and summer generations obtained. The samples of 1986 were collected after the develop under very different conditions. For the T-101 fly release. In July 1986 a sample of pupae was Procida population, therefore, temporal variation in also collected on the adjacent mainland at Monte di allele frequencies was studied over several different Procida. The population from Sardinia was sampled months in four consecutive years (1983—1986) to for four consecutive years (October 1985, September determine if there was a change in local breeding struc- 1986, September 1987, September 1989: two samples) ture and/or in the levels of genetic variation. Adapta- in the Western and Eastern parts of Central Sardinia. tion to overwintering constrains and to the type of host Collections of wild flies from these Mediterranean availability is a function of genetic plasticity of the populations were made by harvesting infested fruits species and may result in genetic differentiation of the (peach, apricot, fig, bitter orange) from the ground Mediterranean populations. beneath trees, and rearing the larvae in the laboratory. Materialsand methods American population. This population was sampled once in February 1989 from coffee berries from a farm Populationsof Ceratitis capitata. Two populations from near Antigua (Guatemala). the African region (Kenya and Reunion Isl.), two populations from the Mediterranean basin (Sardinia Electrophoretic studies. Preparation of samples and and Procida island in the Bay of Naples, Italy) and one electrophoretic procedures are described by Gasperi et American population (from Guatemala) were studied a!. (1991). For each population sample at least 25 indi- according to sampling opportunities. viduals were assayed at the following 27 enzyme loci: Mpi, Est6, Md/i2, Hk2, Est1, Est2, Pgi, Zw, Pgd, Fh, African populations. The Kenya population was repre- Had, Hk1, Id/i, Pgm, Got1, Got2, Ak2, Md/i1, Adh2, sented by four samples of pupae on coffee berries Gpt, Pgk, Me, a-Gpdh, Aox, Ak1, Acon1, Acon,. Stain- collected in August and October 1984 and in April ing for enzyme activities after electrophoresis was and October 1988 on two farms near Nairobi. The based on recipes of Meera Khan (1971) and Harris & population on Reunion Island was sampled four times; Hopkinson (1976). The loci a-Gpdh, Aox, Ak2, were in October and December 1987, near St Denis and in monomorphic in all samples. February 1988 and in January 1990 in St Leu from pupae collected on Mimusops elengi (Sapotaceae). Data analysis. The measurement and testing of hetero- geneity among samples from different geographical Mediterranean populations. Procida is an island in the areas were carried out in different ways. H5, HT, F15, gulf of Naples (Italy) located 2.7 km from the mainland F and FST statistics were computed by the method of and with a total surface of 3.7 km2. On this island Nei & Chesser (1983). The FST values were tested for during the year the density of the medfly population is departure from zero with the method of Workman & oscillatory, with a very low density phase in winter and Niswander (1970). Two ordination approaches have early spring (December—April: 20 flies/ha), a medium been used for electrophoretic data: principal com- density phase in May—July (200 adults/ha) and very ponent analysis and a tree representation. The high densities in August—September (30,000 adults/ha) rationale for applying principal component analysis to (Cirio, 1974). The overwintenng medfly population is allele frequencies is given in Cavalli Sforza & Bodmer composed mainly of larvae utilizing bitter orange as a (1971), and in Guglielmino et a!., (1990). Principal DIFFERENTIATION IN POPULATIONS OF C. CA P1 TA TA 103 component analysis (Sneath & Sokal, 1973) is used to Results examine spatial patterns of association of populations. The method allows one to summarize the total varia- Atree obtained with Cavalli Sforza and Edwards' tion