Assessing the Interfertility of Sexual Chara canescens Populations from Europe: Difficulties and Challenges

Most known populations of the charophyte Chara canescens consist entirely of parthenogenetic reproducing females. Conversely, sexual populations are quite rare. A European project is trying to unravel the degree of genetic exchange between C. canescens populations. As part of this issue, interfertility experiments (cross-fertilisation) of females with males from different sites were planned. Of 72 potential combinations, seven were successfully performed, in five of them oospores were produced, and only supposedly cross-fertilised oospores appeared from one crossing (Sechsmahdlacke’s males, Austria x Pauli Murtas’ females, Italy). Among the main drawbacks faced (i) using specimens directly from the field are the low number of sexual populations in Europe; the long distances between sexual populations to obtain specimens simultaneously for the crossings; the low number of male individuals in the populations; and the difficulties in getting “healthy” cultures in the laboratory; whereas, (ii) by using specimens germinated in the laboratory: the relatively low number of germinated individuals in general; the very low number of germinated male individuals; the long time needed for the germination from the sediment oospores and the asynchronous appearance between populations of different origin; and, finally, the possible sympatric existence of both types of females in the same site, and the impossibility of distinguishing among them visually. Regarding this last issue, sympatric occurrence of C. canescens sexually and parthenogenetically reproducing females is confirmed for the first time in the Las Eras water body (Spain). This meant not knowing whether the oospores formed in the three crossings with females from this site were parthenogenetically or sexually produced. Although the interfertility results remain inconclusive regarding reproductive isolation among distant C. canescens populations, the methodological framework presented here provides a solid foundation for future research.