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Male-killing bacterium in a fifth ladybird beetle, Coleomegilla maculata (Coleoptera:Coccinellidae)
G D Hurst1, T C Hammarton, J J Obrycki
1Department of Entomology, Iowa State University, Ames 50011, USA.
Abstract:
Inherited symbionts which selectively cause the death of male hosts are found widely across the Insecta. Previous studies have shown a single, but different micro-organism to be responsible for male-killing in each taxonomic group studied. We here produce evidence that within a group of insects, the Coccinellidae, there is more than one causal agent of male lethality. We report a novel observation of a male-killing trait in the species Coleomegilla maculata. Six of 26 crosses were found to produce a female-biased sex ratio associated with a low egg hatch-rate. The trait was matrilinearly inherited and was observed to be tetracycline-sensitive. However, tests which indicate the presence of a Rickettsia, previously found to cause male-killing in another member of the Coccinellidae, Adalia bipunctata, proved negative. We therefore conclude that the phenomenon of male-killing is multicausal, within, as well as between, taxonomic groups of the Insecta.
Insights
Male-killing symbionts, inherited microorganisms causing male insect death, are not limited to one cause per species. This study found multiple male-killing agents within the Coccinellidae (ladybug) family.
Area of Science:
- Evolutionary biology
- Insect microbiology
- Genetics
Background:
- Inherited symbionts are known to cause male-killing in various insect groups.
- Previous research identified distinct microorganisms responsible for male lethality in different insect taxa.
- The Coccinellidae family has previously been shown to harbor male-killing symbionts.
Purpose of the Study:
- To investigate the diversity of male-killing agents within the Coccinellidae family.
- To report a novel observation of male lethality in Coleomegilla maculata.
- To determine if the male-killing trait in C. maculata is caused by the same symbiont found in Adalia bipunctata.
Main Methods:
- Performing controlled crosses with Coleomegilla maculata to observe sex ratios and egg hatch rates.
- Assessing the inheritance pattern of the male-killing trait (matrilineal).
- Testing the tetracycline sensitivity of the trait.
- Conducting molecular tests to detect the presence of Rickettsia.
Main Results:
- A female-biased sex ratio and low egg hatch rate, indicative of male-killing, were observed in 6 out of 26 crosses of C. maculata.
- The male-killing trait in C. maculata was confirmed to be matrilinearly inherited and tetracycline-sensitive.
- Molecular tests for Rickettsia, a known male-killing agent in A. bipunctata, were negative in C. maculata.
Conclusions:
- The male-killing phenomenon in Coccinellidae is caused by multiple, distinct agents.
- This finding supports the conclusion that male-killing symbionts are multicausal, both within and between insect taxonomic groups.
- The specific agent responsible for male-killing in C. maculata remains unidentified but is distinct from the Rickettsia found in A. bipunctata.