A KILLING AGENT FOUND IN A NATURAL POPULATION OF <i>DROSOPHILA MELANOGASTER</i>

Abstract

Parasitisms or infections with microorganisms are not rare phenomena in Drosophila. Genetical analyses of a virus responsible for COz sensitivity of flies (L'Heritier 1958) and a spirochete for "sex ratio" agent (Poulson and Sakaguchi 1961) have been performed. The present author found some killing agent for its carrier of Drosophila melanogaster, in the course of isolation of second chromosomes from natural populations in Hiroshima City in 1964, when the carriers were crossed with Cy/Pm flies, the number of F1 offspring decreased appreciably and the segregation of Curly and Plum zygotes was distorted significantly.The cause of the distortion was ascribed to a higher mortality of the Cy-zygotes. Hence, the killing agent was tentatively called Cy-killer (Minamori 1965a, b).Data presented in this paper show that the killing agent may be infective microorganism in the flies, and flies having different genotypes might have different survival rates as a consequence of disturbance by the agent. MATERIALS AND METHODSWhen Cy/Pm females were crossed with several wild males collected from a pickle factory on Tera Street, Hiroshima City in 1961, a segregation ratio between Cy-and Pin-zygotes in the offspring was distorted.Eighteen males (7.11 %) among 253 males collected in summer and five males (1.94 %) among 258 males collected in autumn showed the segregation distortion in their offspring. These males were assumed to carry a killing agent.Second chromosomes were isolated by a mating scheme represented in Figure 1, from these carriers or other non-carriers.On the other hand, these chromosomes were classified into three groups ; lethal (0 %), semilethal (less than 16.7 %) and quasinormal (more than 16.7 %) groups according to the viability of homozygotes in the F4 generation. In the present study, a symbol K is given for the carrier ahead the line number, e. g. K-89, and a symbol F is given for non-carrier, e. g. F-413.The nature manifested by the killing agent was tested clearly by mating the carrier with Cy/Pna or other strains having marker genes.Eight strains, which carry quasinormal second chromosomes homozygously and the killing agent, were used in this experiment.The nature could be measured by the grade of segregation distortion and its magnitude is represented by the frequency of non-Curly flies in the offspring. The number of progeny is also used as a measure for the magnitude of nature.

INCREASING TREND IN FREQUENCIES OF LETHAL AND SEMILETHAL CHROMOSOMES IN A NATURAL POPULATION OF <i>DROSOPHILA MELANOGASTER</i>

“…than expected (50%), the wild male or the wild female from which these offsprings were descended was assumed to be infected since the infective agent kills more Curly zygotes than Plum zygotes (Minamori 1967 In the present study, the chromosomes were classified into 3 categories: sensitive (S), In Table 1 noted that the lethal frequency in 1971 was nearly equal to that obtained in the previous year, while the semilethal frequency increased significantly. The frequency of allelism between lethal genes sampled from each collection is shown in Table 2.…”

“…It was reported that flies carrying a lethal chromosome suffer severer damage by infection with the killing agent than flies carrying a semilethal or quasinormal chromosome; i.e., the viability of lethal heterozygotes was reduced by 10% compared to that of semilethal or of quasinormal heterozygotes when infected (Minamori 1967). …”

“…3) The lethal plus semilethal frequency increased and allelism frequency decreased from early summer to fall in each year. 4) An infective agent was discovered in this population in 1964, and it was recognized to reduce lethal frequencies, but not semilethal frequencies (Minamori 1967;Minamori and Tatsukawa 1968) . 5) An extrachromosomal element denoted by delta was found in this population in 1965 (Minamori 1969) .…”

Extrachromosomal Element Delta in Drosophila melanogaster. IV. Variation and Persistence of Delta-Associating Second Chromosomes in a Natural Population

“…A single wild male or a single daughter of an individual wild female was used for VARIATION OF DELTA-ASSOCIATING CHROMOSOMES 737 crossing to CyIPm females or males. Flies that possessed the Cy chromosome were sensitive to the killing action of the infective agent (Minamori, 1967), but insensitive to the killing action of delta. In contrast, flies that possessed the Pm chromosome were insensitive to the infective agent, but sensitive to delta (Sc).…”

Relation of Infection to Population Structure in Drosophila melanogaster

“…An infection by microorganism was found to occur in natural populations of Drosophila melanogaster. In a previous paper (Minamori, 1967), it was reported that a large per cent of the progeny of infected individuals are killed by the infective agent, and the survival rates vary appreciably according to the fly's genotypes. It is of interest to students of population genetics to determine whether or not the infection has effects on the population structure of flies.…”