THE ACRASINA
163
and the success of certain grafts of slug tips in drawing the cells of other
slugs after them (Raper, 1940b; Bonner, 1950).
Species of Dictyostelium that enter common aggregations build
separate grex at the centre (Raper and Thorn, 1941); and Bonner and
Adams (1958) believe that even in a single strain there is a period of
violent cell mixing as the grex is formed (though it seems possible that
the more newly arrived cells simply surround the earlier ones), and that
this is a manifestation of cells with slightly different properties sorting
themselves out. Thus, as far as contact reactions are concerned, the
Acrasina seem to obey what may be a general rule with cells derived
from multicellular animals (Spiegel, 1954; Moscona, 1960): different
types initially associate rather indifferently during aggregation and
segregate secondarily. This raises the problem of whether this is because
new properties appear or simply because existing ones take so long to
operate. The second alternative may adequately account for the examples just given, for relative changes in position within a stream, which
certainly do occur (Shaffer, 1957b), will have little effect on the total
distribution of cell types in it, compared with that of the same rate of
change within the centre, which is virtually stationary, highly compact,
and does not elongate for a considerable time. Likewise we need not
ascribe the relative fixity of cell position within the lying grex to a change
in properties, but only to the effectiveness of prior sorting; for if the
cells in this stage, whether from one or more strains, are mingled or
grafted together, they again separate to an extent dependent on the
capacity for sorting they displayed at an earlier stage and on the
amount of time available before fruiting bodies are constructed (Raper
and Thorn, 1941; Bonner, 1952; Bonner and Adams, 1958).
Bonner and Adams exchanged centres between aggregations of six
different strains of the three Dictyostelium species known to form common aggregations when grown together (Raper and Thorn, 1941) and
known to use the same acrasin (Shaffer, 1953a); they found three
degrees of compatibility between the strains. In re-examining their
results, it must be emphasized that they cannot be expressed in terms
of compatibility between pairs of strains, because reciprocal grafts behaved differently. If we consider only whether the host would or would
not adhere to the graft to form a common grex (the other difference
studied does not appear to have been a reflection of the properties of the
cell surface), then the results, for all their apparent complexity, may
be expressed very concisely: two strains were 'universal' leaders, that is,
they were followed by all six of them; apart from this, each strain followed
only itself. If these effects are due to specific adhesion, this gives the
extremely interesting result that a cell's back and front must have different
specificity.
163
and the success of certain grafts of slug tips in drawing the cells of other
slugs after them (Raper, 1940b; Bonner, 1950).
Species of Dictyostelium that enter common aggregations build
separate grex at the centre (Raper and Thorn, 1941); and Bonner and
Adams (1958) believe that even in a single strain there is a period of
violent cell mixing as the grex is formed (though it seems possible that
the more newly arrived cells simply surround the earlier ones), and that
this is a manifestation of cells with slightly different properties sorting
themselves out. Thus, as far as contact reactions are concerned, the
Acrasina seem to obey what may be a general rule with cells derived
from multicellular animals (Spiegel, 1954; Moscona, 1960): different
types initially associate rather indifferently during aggregation and
segregate secondarily. This raises the problem of whether this is because
new properties appear or simply because existing ones take so long to
operate. The second alternative may adequately account for the examples just given, for relative changes in position within a stream, which
certainly do occur (Shaffer, 1957b), will have little effect on the total
distribution of cell types in it, compared with that of the same rate of
change within the centre, which is virtually stationary, highly compact,
and does not elongate for a considerable time. Likewise we need not
ascribe the relative fixity of cell position within the lying grex to a change
in properties, but only to the effectiveness of prior sorting; for if the
cells in this stage, whether from one or more strains, are mingled or
grafted together, they again separate to an extent dependent on the
capacity for sorting they displayed at an earlier stage and on the
amount of time available before fruiting bodies are constructed (Raper
and Thorn, 1941; Bonner, 1952; Bonner and Adams, 1958).
Bonner and Adams exchanged centres between aggregations of six
different strains of the three Dictyostelium species known to form common aggregations when grown together (Raper and Thorn, 1941) and
known to use the same acrasin (Shaffer, 1953a); they found three
degrees of compatibility between the strains. In re-examining their
results, it must be emphasized that they cannot be expressed in terms
of compatibility between pairs of strains, because reciprocal grafts behaved differently. If we consider only whether the host would or would
not adhere to the graft to form a common grex (the other difference
studied does not appear to have been a reflection of the properties of the
cell surface), then the results, for all their apparent complexity, may
be expressed very concisely: two strains were 'universal' leaders, that is,
they were followed by all six of them; apart from this, each strain followed
only itself. If these effects are due to specific adhesion, this gives the
extremely interesting result that a cell's back and front must have different
specificity.