What have the Romans ever done for us? The past and future contribution of culture studies to diatom systematics
Mann, David G.; Chepurnov, Victor A.
The first laboratory cultures of diatoms were made in the mid nineteenth century and involved incubating samples of mixed natural populations. Clonal cultures were first reported in 1892. Since then, rough, clonal and axenic cultures have provided many insights into taxonomically relevant aspects of diatom biology, although diatom taxonomy has remained obstinately linked to the study of cleaned silica valves. Relevant discoveries using cultures include diplonty, control of sexuality by cell size, specific and generalized patterns of shape change during size reduction, mechanisms of sexual reproduction (e. g. oogamy v. isogamy), heterothally (dioecy), reproductive isolation between morphologically similar demes (semicryptic species), aspects of phenotypic plasticity, patterns of cell wall and protoplast ontogeny, evolutionary relationships (from molecular sequence data), and population genetics. There is considerable scope for further contributions by culture-based studies in each of these subject areas but we suggest that increased attention should be paid to examining mechanisms of dispersal and gene flow, intrinsic reproductive isolation between allopatric demes, discovery of cryptic species, mate choice, evolution of the karyotype, 'evo-devo' (using molecular phylogenies to reveal how changes in wall morphology and protoplast structure have been brought about through modifications of developmental pathways), and girdle development and function. In addition, there is a great need for simple observations ('natural history') of e. g. growth form, motility, secretions, protoplast structure and dynamics, and mechanisms of sexual reproduction. Without such basic information about diatom structure and function, molecular phylogenies will be largely bald and uninteresting. Major impediments to the development and maintenance of diatom culture collections are (1) the finite life of clones (which has consequences for validation of research) because of size reduction and obligate sexual reproduction, (2) absence of dormant stages and lack of cryopreservation methods, and (3) recalcitrance in culture of many medium- and large-celled benthic species.