Reproductive Biology Promiscuity

The reproductive biology of ladybugs raises several evolutionary problems. Both male and female ladybugs are highly promiscuous. Theoretically, females that produce large and energetically expensive germ cells should mate only often enough to ensure high fertilization rates. This theoretical limitation reflects the energetic and temporal costs of copulation and the possibility of contracting sexually transmitted diseases. Yet females of A. bipunctata mate about 10 times as often as they need to to fertilize all their eggs. Such promiscuity may represent a bet-hedging strategy, or it may afford a means of providing conditions for sperm competition. A hedging strategy addresses the unpredictability of the environments that will face a female's progeny: by mating with a wide variety of different males, she is assured of producing genetically diverse offspring, at least some of which may have genes appropriate to the unknown future habitat. With respect to sperm competition, female ladybugs store the sperm they receive from males in a storage organ called a spermatheca. Thus, after mating with many males, the spermatheca will contain contributions from a number of different males, and these sperm will have to compete for the opportunity to fertilize the eggs.

Female Mate Choice

The study of the reproductive biology of ladybugs has an important place in evolutionary biology, for one of Darwin's mechanisms of evolution was first demonstrated on a ladybug. In addition to natural selection, Darwin argued that some characteristics of some organisms were the result of sexual selection through either male competition or female choice of mates. That females may have a genetically controlled preference to mate with males of a particular genetic type was first demonstrated in A. bipunctata, over a hundred years after the theory was first proposed. In brief, it was shown that some females carry a single gene that is expressed as a preference to mate with melanic rather than nonmelanic males, irrespective of their own color. Subsequently, mating preferences were shown to exist in other species of ladybug.

Apparent Waste of Sperm

Male ladybugs also present some interesting problems. In a single copulation, for example, a male A. bipunctata can transfer to a female up to three sperm packages, or spermatophores. The spermatheca of a female can store about 18,000 sperm. An average spermatophore contains about 14,000 sperm. Therefore, a male that transfers three spermatophores passes more than twice the number of sperm a female can contain. This apparent waste is difficult to comprehend. Possibly by transferring an excess of sperm, the male is indulging in a coarse type of sperm competition, in which sperm in the female's spermatheca from previous matings are flushed out.

Consequences of Promiscuity: Sexually Transmitted Diseases

Not only do both sexes of many species mate many times, but the duration of each copulation is considerable, lasting several hours in many species. This promiscuity has had one obvious consequence: some species of ladybug are infected by sexually transmitted diseases. Such diseases are generally rare in invertebrates, yet both sexually transmitted mites and fungi infect ladybugs. The mite Coccipolipus hippodamiae, which appears to specialize on ladybugs, lives under the elytra, with its mouthparts embedded into the elytra, from which it sucks hemolymph. Mite larvae emerging from eggs produced by the adult females travel to the posterior end of their host before moving onto a new host when the ladybug copulates. A sexually transmitted fungus (in the Laboulbeniales) also occurs.

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