ABSTRACT: Phylogenetic analyses of asymmetry variation offer a powerful tool for exploring the interplay between ontogeny and evolution because a) conspicuous asymmetries exist in many higher metazoans with widely varying modes of development, b) patterns of bilateral variation within species may identify genetically and environmentally triggered asymmetries, and c) asymmetries arising at different times during development may be more sensitive to internal cytoplasmic inhomogeneities compared to external environmental stimuli. Using four broadly comparable asymmetry states (symmetry , antisymmetry , dextral , sinistral ), and two stages at which asymmetry appears developmentally (larval , post-larval ), I evaluated relations between ontogenetic and phylogenetic patterns of asymmetry variation. Among 140 inferred phylogenetic transitions between asymmetry states, recorded from eleven classes in five phyla, directional asymmetry (dextral or sinistral) evolved directly from symmetrical ancestors proportionally more frequently among larval asymmetries. In contrast, antisymmetry, either as an end state or as a transitional stage between symmetry and directional asymmetry, was confined primarily to post-larval asymmetries. The ontogenetic origin of asymmetry thus significantly influences its subsequent evolution. Furthermore, because antisymmetry typically signals an environmentally triggered asymmetry, the phylogenetic transition from antisymmetry to directional asymmetry suggests that many cases of laterally fixed asymmetries evolved via genetic assimilation.
FIG. 1. Phylogenetic variation in the side of the asymmetrical priapium in male phallostethid fishes (from 60, 61), illustrating the evolutionary changes in asymmetry state expected for a post-larval developing trait. The priapium is a bizarre clasping structure, unknown outside the family Phallostethidae (Atherinomorpha), that evolved from highly modified pelvic bones and is used during mating. Numbers in boxes indicate state changes defining each branch.
Table 2. Frequencies of asymmetry-state transitions inferred from phylogenetic analyses of asymmetry variation in the higher Metazoa
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Number of independent clades exhibiting derived state
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Symmetrical Antisymmetric Right/Dextral Left/Sinistral
Ancestral ____________ _____________ _____________ ______________
state L L? P? P L L? P? P L L? P? P L L? P? P
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a) Reliable transitions only *
Symmetrical -- -- -- -- 1 1 25 2 6 7 2 4 5 2
Antisymmetric -- -- -- -- 1 8 7
Right/Dextral 4 5 1 -- -- -- -- 22 3
Left/Sinistral 5 -- -- -- --b) All transitions **
Symmetrical -- -- -- -- 3 1 29 2 2 6 10 3 4 5 3
Antisymmetric -- -- -- -- 1 1 12 14
Right/Dextral 4 5 5 -- -- -- -- 22 3
Left/Sinistral 5 -- -- -- --c) Condensed ***
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Symmetrical Antisymmetric Directional(D+S) Totals
____________ _____________ ________________ ______________
L+L? P+P? L+L? P+P? L+L? P+P? L+L? P+P?
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Symmetrical -- -- 1(3) 26(30) 8(11) 20(24) 9(14) 46(54)
Antisymmetric 0(0) 0(0) -- -- 0(1) 16(27) 0(1) 16(27)
Directional(D+S) 4(4) 0(0) 5(5) 1(5) 22(22) 8(8) 31(31) 9(13)
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L- asymmetry arises in larva, P- asymmetry arises post-larval, L?, P?- precise time asymmetry arises during development not confirmed.
-- transition not relevant.
* includes only inferred state changes without '?' in Appendix (i.e., only those with high confidence).
** includes all inferred state changes from Appendix, regardless of strength of inference.
*** 'Right/Dextral' (D) pooled with 'Left/Sinistral' (S) as 'Directional'; L pooled with L? and P pooled with P?. Numbers outside parentheses from part (a), numbers inside parentheses from part (b). Entries under 'Directional' -> 'Directional' indicate Right->Left transitions pooled with Left->Right transitions.
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(revised Nov. 4, 2002)