Miniaturization of tardigrades (water bears): Morphological and genomic perspectives.

Tardigrades form a monophyletic group of microscopic ecdysozoans best known for surviving extreme environmental conditions. Due to their key phylogenetic position as a subgroup of the Panarthropoda, understanding tardigrade biology is important for comparative studies with related groups like Arthropoda. Panarthropods - and Ecdysozoa as a whole - likely evolved from macroscopic ancestors, with several taxa becoming secondarily miniaturized. Morphological and genomic evidence likewise points to a miniaturized tardigrade ancestor. The five-segmented tardigrade body typically measures less than 1 mm in length and consists of only about 1000 cells. Most organs comprise a relatively small number of cells, with the highest proportion belonging to the central nervous system, while muscles are reduced to a single cell each. Similarly, fully sequenced genomes of three tardigrade species - together with Hox gene expression data - point to extensive modifications, rearrangements, and major losses of genes and even a large body region. Parallels are evident with related ecdysozoans that may have also undergone genomic reductions, such as the nematode Caenorhabditis elegans. We interpret these data together as evidence of miniaturization in the tardigrade lineage, while cautioning that the effects of miniaturization may manifest in different ways depending on the organ or organ system under examination.