A Test of Genital Allometry Using Two Damselfly Species does not Produce Hypoallometric Patterns

Authors

  • Angela Nava-Bolaños,

    1. Departamento de Ecología Evolutiva, Instituto de Ecología, Universidad Nacional Autónoma de México, Ciudad Universitaria, Mexico, D. F., Mexico
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  • Alex Córdoba-Aguilar,

    1. Departamento de Ecología Evolutiva, Instituto de Ecología, Universidad Nacional Autónoma de México, Ciudad Universitaria, Mexico, D. F., Mexico
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  • Roberto Munguía-Steyer

    1. Departamento de Ecología Evolutiva, Instituto de Ecología, Universidad Nacional Autónoma de México, Ciudad Universitaria, Mexico, D. F., Mexico
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Alex Córdoba-Aguilar, Departamento de Ecología Evolutiva, Instituto de Ecología, Universidad Nacional Autónoma de México, Apdo. Postal 70-275, Ciudad Universitaria, 04510, Mexico, D. F., Mexico.
E-mail: acordoba@ecologia.unam.mx

Abstract

It is widely admitted that sexual selection is the responsible force behind genital traits. However, the particular mechanisms of genital evolution are still debated. Recently, studies of genital static allometry in insects have been used to elucidate such mechanisms. Insect genital traits are often reported to show negative allometry (i.e., a slope < 1), which has generated a number of ideas on how genital traits are selected. However, many studies that have inferred selection mechanisms have omitted consideration of the function of genital traits, used unreliable indicators of body size, and only rarely included female genitalia in their analysis. We investigated whether negative allometry operates for genitalia in two damselfly species (Protoneura cara and Ischnura denticollis). Damselflies are suitable for genital allometry tests as their genital function and body size indicators (wing length and head width) are relatively well known and established. First, we show that the aedeagus is used to physically remove sperm from both sperm storage organs (bursa and spermatheca) and that wing length and head width correlate positively with other morphological traits for the two study species. Second, we estimated genital allometry by measuring aedeagal length, vaginal length, bursal volume, and spermathecal volume. Our results indicate no consistent allometric pattern. Allometry for aedeagal length and vaginal width was not the same. Thus, there was no support for a negative allometric relationship. We urge researchers investigating allometry to look directly at how genitalia function rather than inferring function from allometric relationships only.

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