CHROMOSOMAL POLYMORPHISM IN ISOFEMALE LINES AND CAGE POPULATIONS OFDROSOPHILA MELANOGASTER

Abstract

Abstract. -Drosophila melanogaster populations in nature usually carry inversion polymorphisms. When they were transferred to and maintained in the laboratory as large cage populations, frequencies of polymorphic inversions were drastically decreased and finally eliminated. This "cage effect" was observed irrespective of the geographical origin ofthe population or the initial frequency of each inversion. The decrease and elimination of inversions in the cage was not overcome by changing conditions such as medium, temperature, or the number of isofemale lines (40-600) introduced. On the other hand, in the sets of isofemale lines derived from the same geographical origins as the cage populations, each of which was maintained as a small vial population, the inversion frequencies, though decreased from the initial frequencies, were kept at significantly high levels. The cage populations initiated with one or two isofemale lines also maintained the inversion polymorphisms that were as high as vial populations. Natural populations of many Drosophila species are known to be chromosomally polymorphic, the paracentric inversions being the most common (Roberts, 1976). Once the natural populations carrying such polymorphic inversions are transferred to the laboratory and maintained in cages with several thousand flies, they show significant changes in the frequency of inversion chromosomes. Dobzhansky and Pavlovsky (1953) reported gradual changes of inversion frequencies and eventual equilibrium of each inversion in D. pseudoobscura populations. Similarly, using D. ananassae, Tobari and Kojima (1967) and Kojima and Tobari (1969) showed that inversion frequencies eventually reached an equilibrium. On the other hand, Inoue (1979) using D. melanogaster found that all kinds of polymorphic inversions were rapidly eliminated from cage populations.In the present report, we first confirmed the previous observation that inversion polymorphisms were eliminated from cage populations, and then examined various parameters so as to maintain inversion frequencies in laboratory conditions. It was found that inversion polymorphisms were maintained if the populations were kept as the sets of isofemale lines. Moreover, individual isofemale lines in most cases maintained inversion polymorphisms in small vials or even in large population cages. Possible mechanisms for the maintenance of chromosome polymorphisms in natural populations are discussed. MATERIALS AND METHODSD. melanogaster wild flies were collected by sweeping nets or banana traps in various localities (see Tables 1, 2, 5, and 6). Females inseminated in nature were individually kept in glass vials (3 em diameter x 10 em height) to establish isofemale lines. These lines were maintained by mass transfer of about 20-60 flies every 15 days at 25°C. In some experiments flies that were made homozygous for the second chromosomes were employed. These chromosomes were extracted from the natural population by using balanced lethal chromosome stocks with dominant markers, SM] (Cy)/Pm.The cage...

A long-term study on interactions between the Adh and αGpdh allozyme polymorphisms and the chromosomal inversion In(2L)t in a seminatural population of D. melanogaster

“…Contrasting with the maintenance of In(2L)t in the greenhouse population, In(2L)t was eliminated from the Groningen laboratory population 5 years after founding. This is in agreement with the finding that inversions disappear under laboratory conditions ( Van Delden & Kamping, 1989, 1991; Inoue & Watanabe, 1992; Van ‘t Land, 1997). Selectively disadvantageous effects for individuals carrying In(2L)t under standard laboratory conditions are partly explained by the slower development of In(2L)t homokaryotypes ( Van Delden & Kamping, 1989, 1991; Van ‘t Land, 1997).…”

Parallel Evolution of Cold Tolerance Within Drosophila melanogaster

“…Since past work has not found strong effects of laboratory strain maintenance on inversion frequencies (at least for isofemale strains; Inoue and Watanabe 1992 ), and since all populations underwent the same inbreeding process, the significant results given above would appear to represent genuine inversion frequency differences. These findings are concordant with past results on latitudinal inversion clines ( Lemeunier and Aulard 1992 ; Kapun et al 2016 ), and provide evidence for independent decreases in inversion frequency in cool high altitude regions.…”

Temperature‐Related Genetic Changes in Laboratory Populations ofDrosophila subobscura: Evidence against Simple Climatic‐Based Explanations for Latitudinal Clines

“…A potentially important problem in our thermal selection stocks could be the loss of some or many of the naturally segregating polymorphic inversions in Drosophila subobscura in the relatively constant laboratory environmentas seems to happen with Drosophila melanogaster (e.g., Inoue 1979;Inoue et al 1984)-for reasons unrelated to temperature adaptation. We have, therefore, first estimated the chromosomal diversity in the original population at Puerto Montt, in the founding population, and in the thermal stocks after 1 and 2 years since their establishment.…”