Histone Reduction During the Evolution of Multicellularity

Multicellular organisms have evolved multiple times throughout the history of life on Earth. The most recent transition to multicellularity occurred about 200 MYA within the volvocine algae, with phenotypes ranging from unicellular, undifferentiated multicellular and differentiated multicellular all with sequenced genomes. Interestingly, while the genomes of these organisms are very similar showing only a small increase in the number of genes as morphological complexity increases, the number of histones genes is reduced as morphological complexity increases. Compared to Chlamydomonas reinhardtii, unicellular, and Gonium pectorale, colonial multicellular, which have 125 and 133 histones respectively, Volvox carteri, differentiated multicellular, only has 54 histone genes. This reduction in histones is unexpected since increasing in complexity and increasing cellular differentiation should require more histones to regulate the differential expression between tissues. One possible explanation is that this results from a complexity drain where functional demands to a higher levels of biological organization such as the transition to multicellularity, may reduce or render many parts of the lower level of less use and thus loss of those functions is favored by selection (McShea 2002). To investigate the evolutionary history of these genes, phylogenetic, synteny and expression analyses were conducted to dissect the evolutionary history of histones in the volvocales. We found saturating synonymous mutations for H2A, H2B, and H4 histones which strongly suggests that they are undergoing a birth/death process.