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An Evaluation of an Experimental Model for Sympatric Speciation

Method

Method

Origin of Flies

Flies were caught in 1969 at the Wilton Tip, Wellington, using a butterfly net.

Variations in Method

It must be mentioned at the outset that the method of Pimental et al was not adhered to in exact detail. Fishmeal and banana peel were used as attractants for one experimental trial, but an overwhelming preference for fishmeal caused extinction of the banana flies after only one generation of selection. (A summary of the results of this work can be found in Appendix I). After a number of trials with a cage of approximately 500 flies and a variety of potential attractants, ammonium carbonate and amyl alcohol were chosen as attractants. (Ammonium carbonate was dissolved in water at concentrations of 100gm per 100cc cold water and amyl alcohol was diluted to 0.04 strength in a fine suspension with water). For egg-laying, several drops of chemical were placed on a chopped-up cube of yeast-agar-milk powder medium. This gave a physical structure of cracks and crevasses conducive to egg-laying. This substitution of chemicals may have affected the results in that flies were given only two bases for choice, smell and taste, whereas with organic ovipositional attractants there are four bases for choice—smell, taste, structure and sight.

Two other modifications of experimental method were employed. Flies were not marked with model aeroplane paint on the thorax but had a small colour marking applied to one wing tip. The paint used was a page 3 mixture of clear nail varnish and a fine luminous powder. Counting was done with the aid of an ultra-violet lamp (Pal, 1947). This did not appear to affect the behaviour of the flies and avoided the necessity of daily anaesthetisation with carbon dioxide, as flies could be counted in the cages.

To get a clearer idea of trends in selection, different generations of flies were kept separate. Once the females had completed egg-laying, flies of one generation were destroyed and the cages cleaned in preparation for the flies of the next generation. Flies were sexed every 24 hours during hatching and only young virgins were used in the experimental cages. It was felt that this was important as older flies, even if virgins from early sexing, would be more likely to mate immediately on entering the experimental system, probably to a member of their own strain. For each separate generation each strain was limited to 100 males and 100 females, making a total of 400 flies in the system. In all other details the experimental method of Pimentel et al was adhered to. The final experiment ran through only four generations before having to be discontinued through lack of time.

Parameters Recorded

(i)number of eggs laid each day in each generation per phial.
(ii)percentages of eggs laid correctly per day and per generation.
(iii)average percentage migration per strain per generation.

Additional Tests

(a) Testing equal amounts of each attractant

At the end of the experimental work, a test was run on the 200 female flies that would have been selected for the fifth generation. When they were fully developed and ready to lay eggs, the flies were given the choice of equal amounts of each attractant to determine whether or not they chose the correct attractant for their strain in a higher percentage than the attractant of the opposite strain. Each strain was given three egg-laying phials of each attractant since it was felt that this would reduce the chances that an accidental heavy laying in one phial would distort the results (this distortion effect is dealt with in the discussion). The experiment was repeated on four successive days.

(b) Eggs as attractants

This test was performed in an attempt to assess the possibility that freshly laid eggs might act as ovipositional attractants. Sample populations of 200 females flies were offered a choice of two egg laying phials, one having the egg laying medium scattered with freshly laid eggs, the other being untreated egg laying medium. Numbers of eggs used as "attractant" were counted before use in order that they would not weight the results of the experiment.

(c) Strength of chemical vs egg attractants

This test was conducted as a corollary to (b). After Generation 4, flies from the amyl alcohol selected strain were presented with a choice of two egg laying phials, one containing egg laying medium and amyl alcohol, the other egg laying medium and eggs. Similarly the ammonium carbonate selected flies were presented with two choices, either ammonium carbonate on egg laying medium or egg laying medium and eggs.

page 4
Fig. 1 Total and Correct Numbers of Eggs Laid Daily by Each Strain.

Fig. 1 Total and Correct Numbers of Eggs Laid Daily by Each Strain.

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Fig. 2a. Daily Percentage of Eggs Laid Correctly in Each Cage.

Fig. 2a. Daily Percentage of Eggs Laid Correctly in Each Cage.

Fig. 2b. Average Percentage of Eggs Correctly. Laid in Each Cage.

Fig. 2b. Average Percentage of Eggs Correctly. Laid in Each Cage.