A quantitative method to analyse the sexual development of charophytes (Charophyceae): a baseline for phenological studies

Abstract

ABSTRACT Traditionally, gametangium morphology has been considered an important feature in the taxonomy of charophytes (family Characeae), although its dimensions change notably with development. We present a methodological approach based on empirical data to describe the development of charophyte sexual organs. This method was tested in Chara hispida, C. aspera and C. canescens from Mediterranean brackish ponds. Five shoots per species and sampling date were collected on 19 dates during 2015 and observed under a stereomicroscope to measure the external diameter of antheridia, and the length and width of oogonia. Gametangial growth adjusted to a logarithmic model (a first period of rapid increase, followed by a slow growth period), with gametangia sizes reaching species-specific maxima. For oogonia, a logarithmic model also described the length-width relationship: a first period of elongation, followed by a swelling process related to ripening. We applied the derivative function to find the point when length and width increased equally. At greater width values (≥420 µm in C. hispida, ≥330 µm in C. aspera, ≥225 µm in C. canescens), width increased faster than length, and oogonia started to swell. Data from a single date, at the most appropriate moment based on the phenology of each species, also provided a good fit and a similar size threshold to distinguish non-swollen from swollen oogonia. This cost-effective method was valid for the three Chara populations, being particularly useful for phenological studies if used in combination with other criteria. We highlight the importance of standardizing these criteria before addressing relevant ecological questions.