Final answer:
The fact that tunicates have fewer Hox genes than other chordates suggests a genomic simplification likely related to their simpler adult body structure. Hox gene duplication events throughout evolution have resulted in more complex body plans in animals, which aligns with the number of Hox genes present in more complex organisms.
Step-by-step explanation:
The fact that tunicates have only 9 sets of Hox genes while lancelets and all other chordates have 13 sets of Hox genes indicates that there has most likely been a loss or non-duplication of Hox genes in tunicates compared to other chordates. This genomic simplification could be related to their less complex adult body structure.
Hox genes are critical for the embryonic development of animals. They dictate the animal body plan, including the anterior-posterior axis, by determining where and when certain body segments should develop. The number of Hox genes present in an organism can influence the complexity of its body plan, with a greater number generally corresponding to a more complex organism. This is supported by the fact that vertebrates, which are more complex in structure than tunicates, have four sets of Hox gene clusters: Hox-A, Hox-B, Hox-C, and Hox-D.
During animal evolution, Hox genes have undergone duplication events. These duplications have allowed for the development of more complex body plans. As such, the reduction to 9 Hox gene sets in tunicates may reflect their simpler body structure and lifestyle compared to other chordates such as lancelets, which retain all four chordate traits in the adult stage.