Genetic analysis of developmental mechanics in hydra. XI. Mechanism of sex reversal by heterosexual parabiosis

Sex change from female to male occurs at a high rate in hydra when a female polyp is exposed to a male polyp by parabiosis for a limited length of time. The mechanism of this masculinization process was investigated. When a male strain which produces immotile sperm was used to masculinize female strains, the resultant masculinized animals produced only immotile sperm. Evidence was also obtained which indicated that the original female interstitial cells remained in large numbers in the masculinized animals but did not differentiate into gametic cells. These observations suggest that masculinization is achieved not by the hypothetical "masculinizing factor" but by the migration of the male interstitial cells from the male to the female tissue during parabiosis.

Genetic analysis of developmental mechanisms in hydra. X. Morphogenetic potentials of a regeneration-deficient strain (reg-16)

Morphogenetic potentials of hydra tissue involved in head or foot formation were examined in a standard wild-type strain (105) and a mutant strain (reg-16) which has a very low head regenerative but a nearly normal foot regenerative capacity (T. Sugiyama and T. Fujisawa, 1977, J. Embryol. Exp. Morphol. 42, 65-77). Hydra tissue has two types of morphogenetic potentials to control head formation: the potential to form head structure (head-activation potential) and the potential to inhibit head formation (head-inhibition potential). It also has two types of morphogenetic potentials to control foot formation: foot-activation and foot-inhibition potentials. A lateral tissue grafting procedure (G. Webster and L. Wolpert, 1966, J. Embryol. Exp. Morphol. 16, 91-104), was used to examine and compare the relative levels of these potentials in the normal and the mutant strains. The potential levels were examined along the body axis of the intact animals and also in the regenerating animals after head removal. The results obtained show that the potentials involved in head formation are highly abnormal, whereas the potentials involved in foot formation are apparently normal in the mutant strain (reg-16). This suggests that the abnormal potentials are related in some way to, and may be responsible for, the reduced head regenerative capacity in the mutant strain reg-16.

Genetic analysis of developmental mechanisms in hydra. IX. Effect of food on development of a slow-budding strain (L4)

A mutant hydra strain L4 produces buds at a much lower rate than the standard wild-type strain (105) when fed with brine shrimp nauplii (T. Sugiyama and T. Fujisawa, 1979b, Dev. Growth Differ. 21, 361-375). It was found that addition of a small amount of tubifex worm tissue to the normal brine shrimp diet significantly improved the budding rate of L4 but not of 105. Detailed examination and comparison of food effect on various developmental processes in L4 and 105 have provided the following observations. (1) L4 development is strongly affected by food and has significantly lower rates than 105 in all the developmental parameters examined which involve the bud initiation process. In contrast, such effects and differences are not observed in parameters not involving bud initiation. These observations suggest that L4 has a defect(s) in its bud initiating mechanisms and that the expression of this defect is somehow strongly affected by food. (2) Epithelial cells proliferate in L4 nearly as rapidly as in 105 and without food effect. Epithelial cells produced by cell division are mostly utilized to form buds in mature 105 polyps. These cells, however, are not fully utilized for this purpose in L4 polyps which bud very slowly. Instead, they appear to be somehow lost from tissue in these animals.

Genetic analysis of developmental mechanisms in hydra. XII. Analysis of chimaeric hydra produced from a normal and a slow-budding strain (L4)

Chimaeric hydra strains were produced from a normal strain (105) and a naturally-occurring mutant strain (L4) which has a large polyp size, a low budding rate and a high head-inhibition potential. Various properties of the chimaeras were then examined and compared to those of the two parental strains. Hydra tissue consists of three cell lineages: the ectodermal epithelial, the endodermal epithelial and the interstitial cell lineages. Using the methods recently developed by Marcum & Campbell (1978b) and by Wanek & Campbell (1982), six chimaeric strains were produced which contained six different combinations of the three cell lineages from 105 and L4. Evidence obtained from the comparison of the chimaeras and their parental strains indicates that the ectodermal epithelial cell lineage in L4 is primarily responsible for the large polyp size and the low budding rate of this strain, whereas the endodermal epithelial cell lineage is largely, and the interstitial cell lineage is also partially, responsible for the high head-inhibition potential in L4. This suggests that the mechanisms determining the occurrence and location of bud formation and the mechanisms determining the inhibition potential levels are not related to each other (cf. Takano & Sugiyama, 1983; Bode & Bode, 1983). Evidence was also obtained which suggests that the levels of the head-activation and head-inhibition potentials in the chimaeras are determined independent of each other, apparently without the cross-catalytic relationship between them assumed in the Gierer-Meinhardt model (Gierer & Meinhardt, 1972; Meinhardt & Gierer, 1974).

The interstitial cells control the sexual phenotype of heterosexual chimeras of hydra

The three stem cell populations in hydra, the epithelial cells of the ectoderm and endoderm, which make up the body of the hydra, and the interstitial cells, which give rise to nerve cells, nematocytes, and gametes, were tested for their effects on determining the sexual phenotype of individuals. This was done by creating epithelial hydra, which are devoid of interstitial cells and their derivatives, of one sexual type and repopulating them with interstitial cells from individuals of the other sexual type. The resulting heterosexual chimeras were found in all cases to display the same sexual phenotype as that of the interstitial cell donor, indicating this cell type is responsible for the sex of the animal. The epithelial tissue had no influence in determining which gamete type was produced.

Genetic analysis of developmental mechanisms in hydra. VIII. Head-activation and head-inhibition potentials of a slow-budding strain (L4)

Morphogenetic potentials involved in head formation were examined in a hydra strain (L4) which has a very low budding rate and a large polyp size, and the results were compared to those of the normal strain (105). Hydra tissue has two types of antagonistic morphogenetic potentials which are thought to play important roles in head structure formation: the head-activation potential and the head-inhibition potential. Lateral tissue transplantation (Webster & Wolpert, 1966) was used to compare the levels of the two potentials in L4 and the normal strain. It was found that the intact polyp of L4 has a nearly normal or slightly lower head-activation potential but a significantly higher head-inhibition potential than the normal strain. During the course of regeneration after head amputation, the levels of the two potentials in L4 changed in significantly different patterns from those in the normal strain. These abnormalities of the two potentials may be responsible for the developmental and morphological abnormalities in L4. The significance of these observations is discussed in light of the current hydra pattern formation models (Wolpert, Hornchurch & Clarke, 1974; Meinhardt & Gierer, 1974).

Genetic analysis of developmental mechanisms in hydra. VI. Cellular composition of chimera hydra

The homeostatic mechanisms that maintain constant cellular ratios in hydra tissue were studied using mutant and chimeric hydra strains. Mutants having abnormal cellular compositions are isolated through sexual inbreeding of wild hydra, as described in previous papers of this series. Chimeric hydra are produced by making use of a strain (nf-I) which lacks interstitial cells, nerve cells and nematocytes in its tissue. Reintroduction of interstitial cells from other strains (both normal and mutant) into nf-I leads to creation of chimeric strains having epithelial cell lineages from one strain (nf-I) and interstitial cell lineages from others. Analyses and comparisons of the cellular compositions of all these strains revealed that the numbers of nerve or interstitial cells in the chimeras were very similar to (statistically significantly correlated with) those in their interstitial cell donors. Since chimeras and their interstitial cell donors share the same interstitial cell lineages, this suggests that interstitial cells or their derivatives (nerves and nematocytes) play major roles in determining the nerve and interstitial cell levels in the hydra tissue. It is suggested that some form of homeostatic feedback mechanisms are probably involved in regulating the levels of these cell types.

Genetic analysis of developmental mechanisms in hydra. V. Cell lineage and development of chimera hydra

Chimeric hydra were produced by making use of a strain (nf-1) which lacks interstitial cells, nerve cells and nematocytes. This strain arises by spontaneous loss of interstitial cells from its parental strain (sf-1) (Sugiyama & Fujisawa, 1978). Reintroduction of interstitial cells from other strains into nf-1 leads to the creation of chimeric strains that consisted of epithelial cells derived from strain sf-1 and interstitial cells and their derivatives (nerves and nematocytes) from other strains. In chimeras, interstitial or epithelial cells apparently maintain very stable cell lineages; no indication was obtained that suggested interstitial cell differentiation into epithelial cells or dedifferentiation in the opposite direction during the long courses of chimera cultures (up to one year). Developmental characters of chimeras were examined and compared to those of the epithelial cell (sf-1) and the interstitial cell donors. Almost all of the chimera's characters examined (growth rate, budding rate, tentacle numbers, polyp size, regenerative capacity, etc.) closely resembled those of the epithelial cell donor, but not of the interstitial cell donors. This suggests that epithelial cells, rather than interstitial or nerve cells, are the primary determinant of most, if not all, of hydra developmental characters.

Developmental roles of epithelial and interstitial cell lineages in hydra: analysis of chimeras

Chimeric hydra were prepared by recombining epithelial and interstitial cells between 3 strains of hydra of different sizes (maxi, normal, and mini strains). The resulting chimeras generally resembled the epithelial cell parent more than the interstitial cell parent in size, budding rate, tentacle number, and form. This suggests that epithelial cells normally exert considerable influence over hydra morphogenesis. However, the chimeras show some differences ascribable to interstitial cell origin. Furthermore, the 3 original strains, when deprived of interstitial cells, lose their distinguishing size differences. Thus both epithelial and interstitial cells (or interstitial cell derivatives) mutually participate in hydra's development.

Genetic analysis of developmental mechanisms in Hydra. IV. Characterization of a nematocyst-deficient strain

The authors have previously found that mutant hydra strains showing various types of developmental defects can be isolated through sexual inbreeding of wild hydra. One such defective strain, called nem-4, contains virtually no stenoteles, one of the four types of nematocysts present in hydra, in its tentacles. However, stenoteles are present at a normal level in the body column of this strain, and they are turned over also at a normal rate. Grafting experiments between the head region of nem-4 hydra and the body column of wild type hydra (and vice versa) showed that wild type stenotele nematocytes can move into nem-4 tentacles but that nem-4 stenotele nematocytes can not move into the wild type tentacles. These observations suggest that the stenotele nematocytes are produced normally by differentiation from the interstitial cells in the body column of nem-4 hydra, but that they are somehow prevented from migrating into the tentacles in this strain.

Genetic analysis of developmental mechanisms in Hydra. II. Isolation and characterization of an interstitial cell-deficient strain

A mutant strain (nf-I) of Hydra magnipapillata was isolated that contained no interstitial cells, nerve cells or nematocytes. This strain appeared spontaneously in a sexually inbred clone of hydra, and it was recognized by its inability to eat. When force-fed, however, it grew, multiplied by budding and regenerated. In this and in many other respects, nf-I was very similar to the interstitial cell-deficient strain produced by Campbell (1976) by means of colchicine. A chimera strain was produced by the reintroduction of interstitial cells from another strain into nf-I. The properties of nf-I, the chimera and other related strains were examined, and the possible roles that the interstitial cells and the nerve cells play in growth and morphogenesis of hydra are discussed.