Fig. 37. Illustration of the evolution of distorted posterior reduction
patterns in Phacus (see Fig. 36). Taxon names with a given pattern are
shown at the bottom of each box. The most recent ancestor of Phacus
likely possessed a rigid pellicle and undistorted whorled reduction (Wp
52
or 3) (position 1). These cells gave rise to both the deltoid cell shape and
three whorls as seen in Phacus warszewiczii (Wp
53; position 2) and flattened cells with two whorls and clustered strip reduction (Wp
52) similar
to those described here for Phacus pleuronectes and Phacus orbicularis
(center) (position 3). Modification of this latter type of posterior strip reduction resulted in the remaining distortions described in this study. For
instance, twisting of the cell and exaggerated elongation of the caudal
process produced the misshapen whorls and extra terminating strips observed in Phacus longicauda var. tortus (top, center) (position 4). A secondary modification of the strip clusters associated with a prominent
deltoid cell shape resulted in the pattern observed in Phacus triqueter
(upper right) (position 5). Loss of the caudal process and reduction of strip
clusters resulted in the pattern observed in Phacus segretii (right, second
from top) (position 6). Loss of one whorl resulted in the pattern observed
in Phacus acuminatus (right, second from top) (position 7). Comparative
morphology indicates that this condition might be ancestral to the patterns
of strip reduction observed in the oscillans clade (position 8). The absence
of one whorl of strip reduction (like that in P. acuminatus), a reduction in
the overall number of strips (P520, rather than 32), and increased asymmetry of the strip clusters produced the strip reduction patterns found in
the oscillans clade (bottom) (position 8). Twisting of the flattened ancestral cell resulted in the distorted whorls observed here in Phacus pusillus
and Phacus similis (bottom right and center).