Fig. S1.

Resorption classification. Representative images of fused pairs undergoing allogeneic resorption and graded according to severity. Resorption status was graded on a 0 to IV scale: (Bottom) Stage 0 (no resorption observed); (Second From Bottom) stage I (mild resorption = zooid contraction); (Middle) stage II (moderate resorption = zooid contraction, 1° and 2° bud developmental arrest, chimeric blastogeneic asynchrony, residual cardiac activity); (Second From Top) stage III (severe resorption = only remnants visible, no cardiac activity); and (Top) stage IV (complete resorption = one partner has been fully eliminated; only supporting tissue (ampullae, blood vessels, and tunic) remain). (Scale bars: 2 mm.)