Asymmetry in Luticola species

Abstract

Morphological asymmetry in raphid pennate diatoms is detectable not only by valve outline and specific frustule features, but also in their ontogeny. All raphid pennate diatoms exhibit cryptic lateral polarity, which is associated with the asymmetric formation of the valve sides along the apical axis. Differences in the relative widths of primary and secondary valve sides have proven taxonomically important for separating dorsiventral genera within the Cymbellales. However, rigorous studies examining subtle asymmetric differences in valve outline are scarce. In the present geometric morphometric study of 23 Luticola species, the total asymmetry in valve outline of natural populations was decomposed into components of directional asymmetry (DA) and fluctuating asymmetry (FA) to test whether cryptic directional asymmetry has a significant effect on valve outlines across the genus as a whole. The patterns of asymmetric variation provide a view on average asymmetry of populations (evaluated by DA) and individual asymmetric deviations (associated with FA). Subtle, but statistically significant, average irregularities between the primary and secondary sides of Luticola species (i.e., the effect of DA) were found in all but two species. Patterns of DA between valve sides were quite similar in 17 of the analysed species. This likely indicates common developmental patterns in their morphogenesis. The effect of FA, which has often been used as a proxy of developmental instability of populations, was significant in all Luticola species. Since FA scores of individual cells were not correlated with size diminution of most species, FA might reflect the effect of environmental factors but not life cycle changes. The methodological framework presented in this study could be valuable for future work focused on evolutionarily and/or environmentally related shape aberrations and valve variation in pennate diatoms.