Heredity 68 (1992) 557-563 Received 31 July 1991 Genetical Society of Great Britain The evolutionary history of Drosophila buzzatii. XXIV. Second chromosome inversions have differentaverage effects on thorax length ESTEBAN HASSON, JUAN J. FANARA, CONSTANTINA RODRIGUEZ, JUAN C. VILARDI*, OSVALDO A. REIG & ANTONIO FONTDEVI LA t GIBE Departamento de Ciencias BiolOgicas, Facultad de Ciencias Exactas y Naturales. Universidad de Buenos Aires, Ciudad Universitaria Pab. II, 1428 Buenos Aires, Argentina, *Laboratorio de Genética, Departamento de Ciencias B/old gicas, Facultad Ciencias Exactas y Naturales, Universidad de Buenos Aires, Ciudad Universitaria, 1428 Buenos Aires, Argentina and tDepartmento de Genética y Microbiologia, Un/versidad AutOnoma de Barcelona, 08193 Bellaterra (Barcelona), Spain Wedemonstrate a genetic correlation between rearrangements of the second chromosome of D. buzzatii and thorax length, as a measure of body size. The results indicate that 2j and 2jz3 arrange- ments are correlated with large size, whereas 2st arrangement is correlated with small size. Some inversions (2st and 2jz3) show dominant effects and others (2j/f3) exhibit overdominance. These results show that at least 25 per cent of body size variation may be accounted for by the studied karyotypes. The possible integration of the genotypic, phenotypic and fitness levels, and also the possible implications to life-history evolution theories, are discussed. These results suggest that, under moderate to high heritability values, some kinds of chromosomal endocyclic and/or balancing selection may be valuable mechanisms for maintenance of body size variation. Keywords: bodysize, chromosomal inversions,Drosophila buzzatii, fitness, selection, life-history traits.. ness-related morphometric character that is easy to Introduction observe and measure. Body size is one such trait. Although the adaptive basis of chromosomal poly- Several studies have shown that thorax length, as a morphisms in Drosophila is universally accepted, the measure of body size is correlated with adult fitness selective forces involved are only inferred from components, e.g. fecundity (Robertson 1957a, b), indirect evidence from natural and experimental mating success (Butlin eta!., 1982; Taylor eta!., 1987; populations (Sperlich & Pfriem, 1986). Direct Partridge et aL, 1987; Santos et a!., 1988; Taylor & demonstrations of the selective significance of these Kekié, 1988), and longevity (Partridge eta!., 1987). inversion polymorphisms in nature are practically non- Nonetheless, the existence of a relationship between existent (Endler, 1986). In recent years, however, it has the phenotype and the chromosomal rearrangements is been possible to show that selection acts directly on rather controversial. Lande (1979) and John (1983) these polymorphisms through several fitness com- claim that chromosomal repatternings have no effect ponents in natural populations (Anderson et a!., 1979; on the morphological exophenotype. However, a posi- Ruiz eta!., 1986; Salceda & Anderson, 1988; Ruiz & tive relationship has been postulated in some insect Santos, 1989; Santos eta!., 1989; Hasson eta!., 1991). species, although a significant correlation has only been Bearing in mind that selection acts primarily at the demonstrated in a few cases (White & Andrew, 1960; phenotypic level and only secondarily at the genotypic White et a!., 1963; Butlin et aL, 1982; Colombo, one, population biologists have been searching for a fit- 1989). As regards species of Drosophi!a, a relationship between body size and chromosome polymorphism Correspondence: A. Fontdevila, Department of Ecology and Evo- lutionary Biology, University of California, Irvine, California was also reported for the D. subobscura A chromo- 92717, U.S.A. some (Krimbas, 1967) and for the D. pseudoobscura 557 558 E. HASSON ETAL. third chromosome (Thomson, 1971). Additional evi- 2st, respectively. The karyotypes of each collected dence pointing to the effect of chromosomal variation female and her mate were identified by analysing the on morphological characters comes from the corre- polytene chromosomes of 11 progeny larvae. In a few lated response of chromosomal frequencies to pheno- cases both parents were homozygous for the 2j typic artificial selection in D. subobscura (Prevosti, arrangement and these isofemale lines were selected as 1960; Prevosti 1967) and D. pseudoobscura (Druger, homokaryotypes. However, most of the lines were 1962). segregating and the homokaryotypic lines had to be Data on the genetic basis of body size in natural obtained by progeny selection of full sib crosses. This populations is of high relevance to the understanding progeny testing was determined with an accuracy of the evolutionary consequences of natural pheno- higher than 95 per cent by the cytological analysis of 13 typic selection. In D. buzzatii, Santos et al., (1988) and progeny larvae (Barbadilla & Naveira, 1988). The lines Ruiz & Santos (1989) have found that body size is did not attain fixation simultaneously and some lines positively correlated with mating success in a Spanish were kept by mass culturing until all the homokaryo- natural population. These studies have also shown that typic lines were obtained. The maximum number of different karyotypes differ in their mean value for generations before fixation was five. thorax length (Ruiz & Santos, 1989). In this paper we The experimental homokaryotypic stocks were present data showing that non-inbred flies with founded with eggs collected in optimal conditions from different second chromosome karyotypes extracted egg-laying cages, similar to those described by Ruiz et from an Argentinian natural population differ signi- a!. (1986). For each homokaryotypic stock, a total of ficantly in their thorax lengths and we give some infor- 150 mature (4—5 day-old) virgin males and 150 mature mation on the genetic determination of the chromo- virgin females of the same homokaryotypic lines were somal polymorphism on body size. introduced into one of these cages. Each homokaryo- typic line contributed with equal numbers of flies to this founding population. A large sample of collected Materialsand methods eggs was seeded in a culture and this stock was main- tained by mass culturing during four generations The species Drosophila buzzatli before the experiment was initiated. D. buzzatiiis a cactophilic species of the repleta group (mullen complex) that breeds mainly in the rotting cla- Derivationand analysis of different karyotypes dodes of cacti belonging to several Opuntia species. It Atotal of nine (Table 1) homo- and heterogametic probably originated in Argentina (Fontdevila, 1981; crosses were conducted by introducing 100 mature Fontdevila et al., 1982) and has successfully colonized (4—5 day-old) virgin males and 100 mature females of the Mediterranean region (Fontdevila et a!., 1981; the homokaryotypic stocks to egg collecting chambers. Fontdevila, 1981; 1989) and Australia (Barker, 1982) Reciprocal heterogametic crosses were performed in following its natural host plants. Chromosomally, this order to test possible maternal effects. species is highly polymorphic mainly for its second Eggs were allowed to hatch and first instar larvae chromosome (Ruiz eta!., 1985), and fitness differences were seeded in culture vials at optimal density, i.e. 40 have been demonstrated among these arrangements larvae in 6 ml of culture medium, as determined by (Ruiz eta!., 1986; Hasson eta!., 1991). Fanara (1988). Eight to ten replicates were run simul- taneously for each cross. Cultures were maintained at 25°C under continuous light until all adult flies Derivationof homokaryotypic stocks emerged. A random sample of each progeny was Weanalysed flies derived from a collection performed measured 24 h after individual emergences. Thorax in May 1987 at Arroyo Escobar, a locality situated 30 length was measured to the nearest 0.02 5 mm with a km north-west of Buenos Aires city (for a description binocular microscope fitted with an ocular micrometer. of the population, see Fontdevila eta!., 1982). Previous The measure was taken from the anterior margin of the studies showed that this population is polymorphic for thorax to the posterior tip of the scutellum, as laterally four arrangements in the second chromosome (st, j,jz3 viewed. All measurements were done by one of us and jq7) and two in the fourth (Stands) (Fontdevila et (JJF). a!., 1982). A modified formula of David's killed yeast culture Three homokaryotypic stocks involving 17, 14 and medium (David, 1962) was employed. Polytene chro- 4 independently derived chromosomes were obtained mosome slides were obtained following the technique for the second chromosome arrangements: 2j, 2jz3 and described by Fontdevila eta!. (1981). CHROMOSOMAL INVERSION EFFECTS ON DROSOPHILA SIZE 559 Table 1 Mean thorax length and the corresponding Statistical analysis standard error for the different second chromosome karyotypes of D. buzzatii. Sample sizes are indicated in ANOVAwith sex and karyotype as fixed factors and replicates as a random factor nested in karyotypes was parenthesis employed for the analysis of data. The mixed model Number ofMale thorax Female thorax employed can be expressed as: Karyotypes (*) replicateslength (mm) length (mm) X,Jk/=+ af+/3J+FkO)+(a/3)+(aF)k+ EkI st/st 10 1.0273±0.03041.1288±0.0277 (116) (119) whereX,Jklisthe individual value; 1u is the overall mean; j/j 10 1.0396 0.02871.1303 0.0280 a1 is the sex effect, fixed; j3. is the karyotypic effect, (135) (131) 10