Size adjustment occurs during the larval growth of the separated blastomeres of the starfish, Patiria Pectinifera.

Abstract

Starfish embryos derived from blastomeres separated at the early cleavage stage exhibit morphogenesis to form normally shaped, but smaller-sized dwarf bipinnaria larvae. To further understand this developmental capacity, we primarily characterized the morphogenetic processes of separated 2-cell and 4-cell stage blastomeres during the embryonic and larval periods of the starfish, Patiria Pectinifera. Using non-separated blastomeres as the control, we subjected the separated blastomeres to morphological analyses in conjunction with quantitative measurements of the changes in their body sizes with time post-fertilization. Our results were as follows: (i) Blastomeres separated at 2-cell and 4-cell stages synchronously developed into dwarf-sized bipinnaria larvae. (ii) Upon reaching a body size of 500-700 μm, all the bipinnaria larvae originating from the separated blastomeres and controls began to undergo a series of similar organ formation events in preparation for metamorphosis-recognized as the demarcation between the early and late substages of the bipinnaria larval period. (iii) The separated blastomeres became brachiolaria larvae capable of undergoing metamorphosis at differing rates after reaching approximately 1000-1200 μm body sizes, with adult rudiment and sensory organ forming functionally. (iv) The unfed controls and dwarf bipinnaria larvae derived from blastomeres separated at the 4-cell stage arrested their development synchronously without reaching the threshold size required for the latter half of the bipinnaria stage. These results, taken together, suggested that separated blastomeres possess the developmental capacity to become brachiolaria larvae through a shift in morphogenetic regulation from a synchronous growth to size adjustment during the larval period.