Extant Oospores

Oospores represent the diploid zygote of the charophyte plant and are the product of fertilisation of the egg-cell within the oogonium, on female or monoecious thalli, by sperm-cells from male or monoecious thalli. These evolutionary inventions of charophytes are able to cope with and to overcome changed conditions during the water to land transition such as increased UV radiation or drought. These reproductive units are genetic memories of historical environmental conditions through the possibility of reaching a metabolically inert and desiccated state. Oospore morphology varies among genera and species in the Characeae and can be taxonomically important. On the basis of their morphological features, oospores can be used in a wide range of purposes such as to identify past charophyte communities to inform ecological restoration, to inform resurrection ecology studies including the potential to return locally, regionally, and nationally extinct species and to identify the occurrence of extant charophytes at sites even when living charophyte plants are absent (e.g., in drought, or out-of-season). Further, due to their ability to remain dormant and hence disperse through time, as well as through space, oospores are important in the field of resurrection ecology as well as in practical wetland restoration. This chapter also includes recommendations to develop a standardised and comparative method for future oospore analysis as well as for the expansion of the oospore data base.