Dynamics of a cooperative lock-and-key model in genital matching of Sitona Germar (Coleoptera: Curculionidae)

It has been argued that the copulatory structures of the weevil genus Sitona Germar have their equivalents in the opposite sex. Hence, the male intromittent organ represents a species-specific key to unlock the corresponding genital tract of its mate, and the female internal genitalia displays several positions that match the functional armature and surface prominences of the endophallic internal sac from the conspecific male. However, this claim can lead to a major challenge due to the perceived lack of variation in the females copulatory organs across the species of a large genus, Sitona.  Therefore, a special case can be very helpful for modeling an elaborate correspondence of male and female genital traits in this group. One of the most intricating and unusual forms of the genitalia in the tribe Sitonini that is assumed to be the representative of an advanced type of the “lock-and-key” relationship belongs to S. fairmairei. A pair of peculiar “bursal plugs” along with the two slender accessory pouches were revealed inside the female bursal lumen of this species that were considered to operate in concert with the unusual ejaculatory pump and anchoring device of the male transfer apparatus respectively, in the process of insemination. The Modification of the pumping device and the way that the endophallic armature threads along the female spermathecal duct in S. fairmairei were compared to another species, S. cylindricollis. Our results provide evidence on how the biomechanics of copulation can drive the evolution of a “cooperative lock-and-key” isolating mechanism in the genital traits of the weevil genus Sitona, as well as the significance of a clear understanding about the reciprocal morphologies of male and female copulatory organs.