376
classifi ed to be no “true” horizontal groups because originally vertically oriented
ciliary bundles simply follow the curvature of the closest macula margin, gradually leading to an increased horizontal-like orientation (Popper and Platt 1983 )
(Fig. 3 ). Popper and Platt then discussed two alternative hypotheses. First, the
vertical pattern is an ancestral pattern that was retained in otophysans and mormyrids, whereas in the remaining teleosts true horizontal groups evolved at least
seven times independently. The second scenario assumes that the ancestral teleost
condition is the pattern including vertical and horizontal groups and that horizontal groups were lost twice, in otophysans and mormyrids. If the second hypothesis
applies—which is the more parsimonious one—the vertical pattern in otophysans
and mormyrids may have convergently evolved due to similar selection pressures
(Popper and Platt 1983 ).
The vertical pattern is the constant element in each of the fi ve different orientation patterns on the macula sacculi in teleosts (Popper 1981 ) (Fig. 2 ), and the vertical pattern is also found in Dipnotetrapodomorpha, non-teleost actinopterygians
(see above), and Chondrichthyes (Popper and Fay 1977 ). Accordingly, it may further be assumed that the vertical pattern on the macula sacculi is the basic vertebrate pattern on this sensory epithelium (Mathiesen and Popper 1987 ), which did
not experience diversifi cation before the emergence and diversifi cation of the
teleosts.
The fi ve orientation groups can be derived from one another if one either adds
two or three horizontal groups to the vertical pattern (resulting in the standard or the
alternating pattern) or removing the horizontal groups, leading to the vertical pattern (Fig. 2 ). From the standard pattern (1) the dual pattern can be obtained by adding two horizontal groups in the posterior portion and (2) the opposing pattern can
be created by bending the anterior macula downwards in ventral direction while
ciliary bundles retain their horizontal orientation in this area. Alternatively, the standard pattern can emerge from an alternating pattern when one (the most anterodorsal) horizontal group is lost. Only genetic studies could unravel how orientation
groups form during ontogeny, leading to different orientation patterns.
Knowledge about underlying genetic processes of pattern formation is increasing (Duncan and Fritzsch 2012 ; Sienknecht et al. 2014 ) and is likely to shed new
light on the evolution of different orientation patterns in different lineages.
Comparative studies of transcription patterns of candidate genes during ontogenetic
development of maculae in related species with and without ancillary auditory
structures and in non-teleost actinopterygians and teleosts could enlighten whether
and to what extent genetic pathways of ciliary bundle orientation pattern formation
differ. This approach would also provide information on how genes involved in pattern formation evolved in different species and more generally in actinopterygians.
Such studies could use two model organisms as starting points, making use of the
profound knowledge of the genetic background in Danio rerio (Whitfi eld et al.
1996 , 2002 ; Nicolson 2005 ), an otophysan with a vertical pattern on the macula
sacculi, and Oryzias latipes (Hochmann et al. 2007 ), a non-otophysan that very
likely displays the standard pattern.
T. Schulz-Mirbach and F. Ladich
classifi ed to be no “true” horizontal groups because originally vertically oriented
ciliary bundles simply follow the curvature of the closest macula margin, gradually leading to an increased horizontal-like orientation (Popper and Platt 1983 )
(Fig. 3 ). Popper and Platt then discussed two alternative hypotheses. First, the
vertical pattern is an ancestral pattern that was retained in otophysans and mormyrids, whereas in the remaining teleosts true horizontal groups evolved at least
seven times independently. The second scenario assumes that the ancestral teleost
condition is the pattern including vertical and horizontal groups and that horizontal groups were lost twice, in otophysans and mormyrids. If the second hypothesis
applies—which is the more parsimonious one—the vertical pattern in otophysans
and mormyrids may have convergently evolved due to similar selection pressures
(Popper and Platt 1983 ).
The vertical pattern is the constant element in each of the fi ve different orientation patterns on the macula sacculi in teleosts (Popper 1981 ) (Fig. 2 ), and the vertical pattern is also found in Dipnotetrapodomorpha, non-teleost actinopterygians
(see above), and Chondrichthyes (Popper and Fay 1977 ). Accordingly, it may further be assumed that the vertical pattern on the macula sacculi is the basic vertebrate pattern on this sensory epithelium (Mathiesen and Popper 1987 ), which did
not experience diversifi cation before the emergence and diversifi cation of the
teleosts.
The fi ve orientation groups can be derived from one another if one either adds
two or three horizontal groups to the vertical pattern (resulting in the standard or the
alternating pattern) or removing the horizontal groups, leading to the vertical pattern (Fig. 2 ). From the standard pattern (1) the dual pattern can be obtained by adding two horizontal groups in the posterior portion and (2) the opposing pattern can
be created by bending the anterior macula downwards in ventral direction while
ciliary bundles retain their horizontal orientation in this area. Alternatively, the standard pattern can emerge from an alternating pattern when one (the most anterodorsal) horizontal group is lost. Only genetic studies could unravel how orientation
groups form during ontogeny, leading to different orientation patterns.
Knowledge about underlying genetic processes of pattern formation is increasing (Duncan and Fritzsch 2012 ; Sienknecht et al. 2014 ) and is likely to shed new
light on the evolution of different orientation patterns in different lineages.
Comparative studies of transcription patterns of candidate genes during ontogenetic
development of maculae in related species with and without ancillary auditory
structures and in non-teleost actinopterygians and teleosts could enlighten whether
and to what extent genetic pathways of ciliary bundle orientation pattern formation
differ. This approach would also provide information on how genes involved in pattern formation evolved in different species and more generally in actinopterygians.
Such studies could use two model organisms as starting points, making use of the
profound knowledge of the genetic background in Danio rerio (Whitfi eld et al.
1996 , 2002 ; Nicolson 2005 ), an otophysan with a vertical pattern on the macula
sacculi, and Oryzias latipes (Hochmann et al. 2007 ), a non-otophysan that very
likely displays the standard pattern.
T. Schulz-Mirbach and F. Ladich