Enlarge or die! An auxospore perspective on diatom diversification

Abstract

Sexual reproductive cells and processes are highly conserved in a wide range of organisms. A specialized, expandable zygote called the auxospore is a cell known only in diatoms. Auxospores perform two critical functions in the life cycle of most diatoms: (1) as in other sexual organisms, they confer the benefits of genetic recombination to diatom progeny, and (2) restore larger cell size and allow a new cycle of vegetative cell size diminution to commence. Because of this dual importance in the diatom life cycle, the auxospore is strongly conserved, in contrast to the astounding diversity of forms among diatom vegetative cells. The auxospore is a complex cell which develops in a specific sequence of stages with a variety of characteristic structures recognizable across all diatoms. With only two major blueprints for auxospore growth and development (isodiametric and anisodiametric) and a number of highly conserved associated structures, studying this aspect of the diatom life cycle may reveal much about their deep evolutionary history. In the evolutionarily more recent arrivals (i.e., pennates), additional reproductive characters may be informative, e.g., mode of gamete motility, anisogamy vs. isogamy. We argue that combined with the widely varying but taxon-specific aspects of diatom gametogenetic processes, auxospores are excellent tools for cross group comparisons that provide insight into their evolutionary relationships. Expanding our dataset from 21 genera presented in 2004 to 65 polar centrics, nonpolar centrics, and araphid pennates, we repostulate that the evolutionarily informative capacity of auxospore development stands on its own.