5.3. PERTURBATION EXPERIMENTS OF GASTROVASCULAR PHYSIOLOGY
may be regulated directly and the other may be a consequence of the change
in the regulated event. Consider, for example, the spacing of polyps along
stolons. It has long been established that the number of polyps per unit stolon
length is roughly constant for a given clone (Braverman 1963). If redox state,
for instance, regulates stolon tip morphogenesis, then the acceleration of
polyp morphogenesis may be a consequence of the greater total length of
stolon (that is, there is no change in the relative investment in polyps and
stolons) . A simple explanation for the covariation between polyp number
and stolon length or tip number is that polyp spacing may be related to flow.
At some distance from a polyp another polyp is needed to continue transport
of fluid to the growing margin. In other words, fluid exported into the stolon
will travel only so far before being stopped by shear forces along the wall
of the stolon. Thus, one might expect that greater forces exerted by polyps
when pumping (or less resistance to flow) should result in greater spacing
between them. Evidence exists that is consistent with this hypothesis (Buss,
in press). As predicted by the equation balancing pressure and shear forces,
Bussdetected a linear relationship between polyp size (assumed to be linearly
related to the pressure polyps can exert) and twice the stolon length between
polyps divided by stolon radius (measured during phase 3).
The complex relationship between redox state, flow, and morphology
is further demonstrated by considering Hydractinia spp. In Hydractinia,
treatment with DNP shifts the redox state towards oxidation without diminishing flowrates (Blackstone and Buss 1993). In this case, runners maintained
a low rate of polyp bud and stolon tip formation (i.e. continued as runners), whereas sheet colonies (in which flowto stolon tips increased) showed
a decline in rates of bud and tip formation (i.e. subsequently developed as
runners). What appears particularly important in the case of Hydractinia is
how the vascular architecture (the sizes and arrangement of stolons) constrains the distribution and hydrodynamic characteristics of flow within
a colony.
Evidence from experiments manipulating the sizes and arrangement
of stolons supports the hypothesis that vascular architecture determines
the characteristics of flow which regulate further morphological development (Dudgeon and Buss 1996) . Runner colonies are characterized by highly
variable stolon diameters, whereas sheet colonies exhibit little variation in
diameters of stolons . Thus, runner colonies have an unequal distribution of
flow to the growing periphery, whereas sheet colonies have equivalent volumes of flow delivered to the growing tips. These flow patterns reinforce
their respective growth trajectories, as in a self-maintaining system. Surgical manipulation of stolon connections that change flow patterns in the
gastrovascular system changes the subsequent morphological trajectory. Diminishing variation in flowrate among stolons within runner colonies results
in subsequent sheet growth. Conversely, increased variation in flow among
stolons within sheet colonies results in subsequent runner growth . The DNP
experiment of Blackstone and Buss (1993) using Hydractinia can be explained
in the context of this model : the effect ofDNP on the transport of fluid was to
diminish the energy available to pump fluid and to make the delivery of fluid
to stolon tips inconsistent and highly variable, thereby initiating a runner
growth trajectory.
In summary, the behavior of polyps, the redox state of cells, and the
architecture of the vascular system provide information to cells lining the
157
may be regulated directly and the other may be a consequence of the change
in the regulated event. Consider, for example, the spacing of polyps along
stolons. It has long been established that the number of polyps per unit stolon
length is roughly constant for a given clone (Braverman 1963). If redox state,
for instance, regulates stolon tip morphogenesis, then the acceleration of
polyp morphogenesis may be a consequence of the greater total length of
stolon (that is, there is no change in the relative investment in polyps and
stolons) . A simple explanation for the covariation between polyp number
and stolon length or tip number is that polyp spacing may be related to flow.
At some distance from a polyp another polyp is needed to continue transport
of fluid to the growing margin. In other words, fluid exported into the stolon
will travel only so far before being stopped by shear forces along the wall
of the stolon. Thus, one might expect that greater forces exerted by polyps
when pumping (or less resistance to flow) should result in greater spacing
between them. Evidence exists that is consistent with this hypothesis (Buss,
in press). As predicted by the equation balancing pressure and shear forces,
Bussdetected a linear relationship between polyp size (assumed to be linearly
related to the pressure polyps can exert) and twice the stolon length between
polyps divided by stolon radius (measured during phase 3).
The complex relationship between redox state, flow, and morphology
is further demonstrated by considering Hydractinia spp. In Hydractinia,
treatment with DNP shifts the redox state towards oxidation without diminishing flowrates (Blackstone and Buss 1993). In this case, runners maintained
a low rate of polyp bud and stolon tip formation (i.e. continued as runners), whereas sheet colonies (in which flowto stolon tips increased) showed
a decline in rates of bud and tip formation (i.e. subsequently developed as
runners). What appears particularly important in the case of Hydractinia is
how the vascular architecture (the sizes and arrangement of stolons) constrains the distribution and hydrodynamic characteristics of flow within
a colony.
Evidence from experiments manipulating the sizes and arrangement
of stolons supports the hypothesis that vascular architecture determines
the characteristics of flow which regulate further morphological development (Dudgeon and Buss 1996) . Runner colonies are characterized by highly
variable stolon diameters, whereas sheet colonies exhibit little variation in
diameters of stolons . Thus, runner colonies have an unequal distribution of
flow to the growing periphery, whereas sheet colonies have equivalent volumes of flow delivered to the growing tips. These flow patterns reinforce
their respective growth trajectories, as in a self-maintaining system. Surgical manipulation of stolon connections that change flow patterns in the
gastrovascular system changes the subsequent morphological trajectory. Diminishing variation in flowrate among stolons within runner colonies results
in subsequent sheet growth. Conversely, increased variation in flow among
stolons within sheet colonies results in subsequent runner growth . The DNP
experiment of Blackstone and Buss (1993) using Hydractinia can be explained
in the context of this model : the effect ofDNP on the transport of fluid was to
diminish the energy available to pump fluid and to make the delivery of fluid
to stolon tips inconsistent and highly variable, thereby initiating a runner
growth trajectory.
In summary, the behavior of polyps, the redox state of cells, and the
architecture of the vascular system provide information to cells lining the
157