Discovery of a complex sclerotized sperm pump in Monotomidae (Coleoptera): Functional morphology and phylogenetic value.

Abstract

In insects, a long ductus ejaculatorius surrounded by a muscular sheath transfers seminal fluids and sperms from the testes into the male copulatory organ, the aedeagus. In several unrelated coleopteran families, a section of the ductus has evolved to form a specialized pumping organ. The most complex forms of this "sperm pump" was known in Lepiceridae and two subfamilies of evolutionary distant Staphylinidae; in these groups two sclerotized funnels situated on ends of the sperm pump provide attachment sites for a remarkably robust compressor muscle. A sperm pump with two funnels and a previously unknown spiral ridge, representing the most complex sperm pumping apparatus among Coleoptera, is for the first time reported to occur in the superfamily Cucujoidea, family Monotomidae. The compressor muscle is particularly large, and the pump shows features more advanced than those in any other beetle. Not the entire section between funnels is compressed, but only a pair of specialized thin-walled, unsclerotized and resilin-rich regions within the pump. The spiral ridge stores mechanical energy to decompress the pump when the muscle relaxes, and provides a sclerotized scaffold for the entire structure. The pumps do not occur in all Monotomidae, but seem to be restricted to the subfamily Monotominae. Two distinct forms were found, each in a different tribe (with one notable exception; Mimemodes having a sperm pump typical of Monotomini, not Europini). Moreover, the shapes of the pumps were found to be species-specific. The phylogenetic value of this structure is discussed, as sperm pumps may help defining monophyletic lineages within this poorly studied family. An evolutionary scenario for a development of sperm pumps in Coleoptera from an undifferentiated ductus ejaculatorius to the most complex "monotomid form" is proposed, assuming several distinct stages exemplified by structures found in other beetle families.