MINELLI, A.: Homology, limbs and genitalia

Similarities in genetic control between the main body axis and its appendages have been generally explained in terms of genetic co-option. This extends to the parallels in the patterning of arthropod and vertebrate appendages, which have been explained either invoking a common ancestor already provided with patterned body outgrowths, or as the effect of independent recruitment in limb patterning of single genes or genetic cassettes originally used for purposes other than axis patterning. I have suggest instead that these body appendages are evolutionarily divergent duplicates (paramorphs) of the main body axis. The same interpretation may also apply to genitalia, irrespective of whether they are modified limbs, or directly derive ex novo from a duplication of the main body axis. But, are all metazoan limbs and genitalia homologous? Are limbs homologous to genitalia (in vertebrates, or in other phyla)? The answer to these question is never a simple yes or no. The possibility for an animal to express a complex organ ectopically demonstrates that positional and special homology can be evolutionarily uncoupled. The concept of body appendages (limbs and genitalia) as paramorphs of the main body axis eliminates the requirement for the last common ancestor of limb-bearing animals to have been provided with limbs. Examples of a specified axial position being alternatively used for genital and non-genital structures within the same phylum include a mid-body level hotspot corresponding to the millipede male gonopods, the dragonfly secondary penis and the centipede “mid-body anomaly.” Also the transition from paired to unpaired appendage (either limb or genitalia) is not necessarily large, as demonstrated by the insect genitalia (paired in mayflies, unpaired in the remaining pterygotes) as well as by the celacanth fins.