Functional morphology of spermatophores and female genitalia in bothriurid scorpions: genital courtship, coercion and other possible mechanisms

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Abstract
Scorpions use sclerotized spermatophores for sperm transfer. The family Bothriuridae has one of the most complex scorpion spermatophores, with a distinct capsule and lamella. This study evaluates the viability of hypotheses relating to the evolution of animal genitalia that attempt to explain the functional morphology observed in spermatophores and female genitalia of some members of this scorpion family. These hypotheses are: female choice (internal courtship), conflict of interests (morphologically forced insemination) and lock-and-key (interspecific mechanical incompatibility). Observations of the mating sequences of 13 bothriurid and one buthid scorpion were recorded. The histology of female atrium was analysed. The data suggest that: (1) a genital copulatory courtship (sexual stimulation through male genitalia) could be a widespread mechanism among the capsules of the studied spermatophores; the wall of the female genital atrium in Brachistosternus as well as of other genera is usually rubbed by spines, tubercles or capsular lobes of spermatophores; the female genital atrium is relatively soft, elastic and uniform among different species; (2) morphological coercion might be admissible only when capsular lobes are very well developed, as females will have difficulty in disengaging by themselves from the large lobes if they attempt to interrupt sperm transfer; this mainly occurs in Bothriurus bonariensis; (3) observations of matings among closely related species indicate that the lock-and-key hypothesis is unable to explain spermatophore morphology in the studied species; however a certain degree of mechanical incompatibility occurs during interspecific mating between two sister species (B. bonariensis and B. chacoensis) owing to differences in capsular lobes and post-insemination behaviour; (4) these hypotheses might not be directly applied to simple spermatophores of the Buthidae Zabius fuscus. The results indicate that even though one genital mechanism can be more frequent than others, they can also exist together, not only in different genera but also in a single species (e.g. simultaneous occurrence of genital sexual stimulation and genital coercion in B. bonariensis). Indirectly these data suggest that the hypotheses most frequently quoted to explain genital morphology are not mutually exclusive.