Une interprétation évolutive de la gamétogenèse animale

Abstract

Motility is an essential property of all animal germ cells. Gametogenesis can be divided into three main phases with respect to germ cell motility. Primordial germ cells migrate into the gonadal rudiments. A long captive phase follows, during which the germ cells differentiate into ova and spermatozoa. Mature germ cells are finally released from the gonads. In many animal groups they are brought to the exterior by genital ducts. The authors propose an evolutionary interpretation of gametogenesis. They assume that the earliest animals already had a germ line, but no gonads or sex ducts. The three phases of germ cell life then reflect three steps in the evolution of the animals' reproductive system. The first phase corresponds to an early evolutionary stage, when germ cells were not included in a gonad. The intra-gonadal phase corresponds to a later stage, when animals acquired specialized organs to harbor and protect their reproductive cells. The final stage corresponds to the most recent evolutionary innovation. Mature sex products are guided outside by specialized ducts deriving from, or connected to the excretory system.