In apple maggot flies, Rhagoletis pomonella, adults and larvae feed on very spec
ID: 90813 • Letter: I
Question
In apple maggot flies, Rhagoletis pomonella, adults and larvae feed on very specific host plants. There are at least three genetically distinct lineages, one of which specializes on hawthorn trees, another specializes on apples, and the third specializes on cherries. They are pretty good fliers, easily capable of moving several kilometers a day. They can also mate with each other, and the hybrids are fertile. In the example shown below, there are only apple or hawthorn trees available for the maggot flies. Assume that apple specialists feed and mate primarily on apples, and that hawthorn specialists feed and mate primarily on hawthorn fruits. (Sometimes, though, if it were available, an apple specialist might feed on a hawthorn, and a hawthorn specialist might feed on an apple.) Adult flies emerge a few weeks before their preferred fruit matures, fly around for awhile, then lay eggs on their preferred host (where their larvae feed and develop). What are the main barriers to gene flow between these two races? What, if any, is the importance of timing of fruit maturation in this scenario? How would you expect changes in timing of fruit maturation to affect the likelihood that hybrids would be formed? Assume that the apple and hawthorn races were good biological species? How could you distinguish whether they speciated sympatrically or allopatrically?Explanation / Answer
Hawthorns are native to America and apples were introduced to America by immigrants. So, the ancestors of apple maggot flies laid their eggs only on hawthorns. Today, these flies lay eggs on hawthorns and domestic apples.
Females flies lay their eggs on the type of fruit they grew up and males tend to look for mates on the type of fruit they grew. So hawthorn flies generally end up mating with other hawthorn flies and apple flies generally end up mating with other apple flies. This reduced the gene flow between parts of the population that mate on different types of fruit. This shift of flies from hawthorns to apples is called as sympatric speciation.
The diagram represents the timing of fly emergence and fruit ripening. Adult flies emerge reproduce before fruits are mature. Apple fruits ripen approximately 1 month earlier than hawthorn fruits, but there is overlap at the end of the apple fruiting season and the beginning of the hawthorn fruiting season. The timing of fruit maturation is important as female flies lay their fertilised eggs in the ripe fruit only.
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