Sex determination in an echiuroid worm: bonellia

When developmental mechanics was first formulated, some of the obvious variables to manipulate were the temperature and media in which embryos were developing. These early studies initiated several experimental programs on the effects of the environment on development. For instance, Baltzer (1914) showed that the sex of the echiuroid worm Bonellia viridis depended on where the Bonellia larva settled. The female Bonellia worm is a marine, rock-dwelling animal, with a body about 10 cm long (Figure 3.1). She has a proboscis that can extend over a meter in length. The male Bonellia, however, is only 1–3 mm long and resides within the uterus of the female, fertilizing her eggs. Baltzer showed that if a Bonellia larva settles on the seafloor, it becomes a female. However, should a larva land on a female's proboscis (which apparently emits chemical signals that attract the larva), it enters the female's mouth, migrates into her uterus, and differentiates into a male. Thus, if a larva lands on the seafloor, it becomes female; if it settles on a proboscis, it becomes male. Baltzer (1914) and Leutert (1974) were able to duplicate this phenomenon in the laboratory, incubating larvae in either the absence or presence of adult females (Figure 3.2).

Figure 3.1

Sexual dimorphism in Bonellia viridis. The body of the mature female is about 10 cm in length, but the proboscis can extend up to a meter. The body of the symbiotic male is a minute 1–3 mm in length. While the body of the adult female is buried (more...)

Figure 3.2

In vitro analysis of Bonellia sex determination. Larval Bonellia were placed either in normal seawater or in seawater containing fragments of the female proboscis. A majority of the animals cultured in the presence of the proboscis fragments became males, (more...)

Sex determination in a vertebrate: alligator

Recent research has shown that the effects of the environment on development can have important consequences. Such research has shown that the sex of the alligators, crocodiles, and many other reptiles depends not on chromosomes, but on temperature. After studying the sex determination of the Mississippi alligator both in the laboratory and in the field, Ferguson and Joanen (1982) concluded that sex is determined by the temperature of the egg during the second and third weeks of incubation. Eggs incubated at 30°C or below during this time period produce female alligators, whereas those eggs incubated at 34°C or above produce males. (At 32°C, 87% of the hatchlings were female.) Moreover, nests constructed on levees (close to 34°C) give rise to males, whereas nests built in wet marshes (close to 30°C) produce females. These findings are obviously important to wildlife managers and farmers who wish to breed this species; but they also raise questions of environmental policy. The shade of buildings or the heat of thermal effluents can have dramatic effects on the sex ratios of reptiles. We will discuss the mechanisms of temperature-dependent sex determination further in Chapter 17.

Norms of reaction

Another program of environmental developmental biology concerns how the embryo adapts to its particular environment. August Weismann (1875) pioneered the study of larval adaptations, and recent research in this area has provided some fascinating insights into how an organism's development is keyed to its environment. Weismann noted that butterflies that hatched during different seasons were colored differently, and that this season-dependent coloration could be mimicked by incubating larvae at different temperatures. Phenotypic variations caused by environmental differences are often called morphs. One example of such seasonal variation is the European map butterfly, Araschnia levana, which has two seasonal phenotypes so different that Linnaeus classified them as two different species (van der Weele 1995). The spring morph is bright orange with black spots, while the summer form is mostly black with a white band (Figure 3.3). The change from spring to summer morph is controlled by changes in both day length and temperature during the larval period. When researchers experimentally mimic spring conditions, summer caterpillars can give rise to “spring” butterflies (Koch and Buchmann 1987; Nijhout 1991).

Figure 3.3

Two morphs of Araschnia levana, the European map butterfly. The summer morph is represented at the top, the spring morph at the bottom. In this species, the phenotypic differences are elicited by differences in day length and temperature during the larval (more...)

Another dramatic example of seasonal change in development occurs in the moth Nemoria arizonaria. This moth has a fairly typical insect life cycle. Eggs hatch in the spring, and the caterpillars feed on young oak flowers (catkins). These larvae metamorphose in the late spring, mate in the summer, and produce another brood of caterpillars on the oak trees. These caterpillars eat the oak leaves, metamorphose, and mate. Their eggs overwinter to start the cycle over again next spring. What is remarkable is that the caterpillars that hatch in the spring look nothing like their progeny that hatch in the summer (Figure 3.4). The caterpillars that hatch in the spring and eat oak catkins (flowers) are yellow-brown, rugose, and beaded, resembling nothing else but an oak catkin. They are magnificently camouflaged against predation. But what of the caterpillars that hatch in the summer, after all the catkins are gone? They, too, are well camouflaged, resembling year-old oak twigs. What controls this difference? By doing reciprocal feeding experiments, Greene (1989) was able to convert spring caterpillars into summer morphs by feeding them oak leaves. The reciprocal experiment did not turn the summer morphs into catkin-like caterpillars. Thus, it appears that the catkin form is the “default state” and that something induces the twiglike morphology. That something is probably a tannin that is concentrated in oak leaves as they mature.

Figure 3.4

Two morphs of Nemoria arizonaria. (A) Caterpillars that hatch in the spring eat oak catkins and develop a cuticle that resembles these flowers. (B) Caterpillars that hatch in the summer (after the catkins are gone) eat oak leaves. These caterpillars develop (more...)

Embryologists have emphasized that what gets inherited is not a deterministic genotype, but rather a genotype that encodes a potential range of phenotypes. The environment is often able to select the phenotype that is adaptive for that season or habitat. This continuous range of phenotypes expressed by a single genotype across a range of environmental conditions is called the reaction norm (Woltereck 1909; Schmalhausen 1949; Stearns et al. 1991; Schlichting and Pigliucci 1998). The reaction norm is thus a property of the genome and is also subject to selection. Different genotypes will be expected to differ in the direction and amount of plasticity that they are able to express (Gotthard and Nylin 1995; Via et al. 1995).

Protection of the egg by sunscreens and repair enzymes

The survival of embryos in their environments poses major problems. Indeed, as Darwin clearly noted, most eggs and embryos fail to survive. A sea urchin may broadcast tens of thousands of eggs into the seawater, but only one or two of the resulting embryos will become adult urchins. Most become food for other organisms. Moreover, if the environment changes, embryonic survival may be increased or decreased dramatically. For instance, many eggs and early embryos lie in direct sunlight for long periods. If we lie in the sun for hours without sunscreen, we get sunburn from the ultraviolet rays of the sun; this radiation is harmful to our DNA. How can eggs survive all those hours of constant exposure to the sun (often on the same beaches where we sun ourselves)? First, it seems that many eggs have evolved natural sunscreens. The eggs of many marine organisms possess high concentrations of mycosporine amino acid pigments, which absorb ultraviolet radiation (UV-B). Moreover, just like our melanin pigment, these pigments can be induced by exposure to UV-B radiation (Jokiel and York 1982; Siebeck 1988). The eggs of tunicates are very resistant to UV-B radiation, and much of this resistance comes from extracellular coats that are enriched with mycosporine compounds (Mead and Epel 1995). Similarly, Adams and Shick (1996) experimentally modulated the amount of mycosporine amino acids in sea urchin eggs and found that embryos from eggs with more of these compounds were better protected from UV damage than embryos with less. So some of the beautiful pigments found in marine eggs have a very practical function.

The possibility exists that the recent global decline of amphibian populations may be caused by increasing amounts of UV-B reaching the Earth's surface. Populations of amphibians in widely scattered locations have been drastically reduced in the past decade, and some of these species (such as the golden toad of Costa Rica) have recently become extinct. While no single cause for these declines has been identified (and the extinctions may be due to the convergence of more than one cause), the fact that they have occurred in undisturbed areas and throughout the world has prompted considerations of global phenomena (see Phillips 1994).

Blaustein and his colleagues (1994) have looked at levels of photolyase, an enzyme that repairs UV damage to DNA by excising and replacing damaged thymidine residues (Figure 3.5), in amphibian eggs and oocytes. Levels of photolyase varied 80-fold among the tested species and correlates with the site of egg laying. Those eggs more exposed to the sun had higher levels of photolyase (Table 3.1) These levels also correlated with whether or not the species was suffering population decline. The highest photolyase levels were in those species (such as the Pacific tree frog, Hyla regilla) whose populations were not seen to be in decline. The lowest levels were seen in those species (such as the Western toad, Bufo boreas, and the Cascades frog, Rana cascadae) whose populations had declined dramatically.

Figure 3.5

The photolyase reaction. Photolyase repairs thymidine dimers caused by ultraviolet radiation. The bonds (red lines) linking the two thymidines (colored molecules) are shorter than the normal spacing between the bases and therefore distort the DNA in that (more...)

Table 3.1

Photolyase activity correlated with exposure of eggs to ultraviolet radiation in 10 amphibian species.

Blaustein and his colleagues tested whether or not UV-B could be a factor in lowering the hatching rate of amphibian eggs. At two field sites, they divided the eggs of each of three amphibian species into three groups (Figure 3.6). The first group developed without any sun filter. The second group developed under a filter that allowed UV-B to pass through. The third group developed under a filter that blocked UV-B from reaching the eggs. For Hyla regilla, the filters had no effect, and hatching success was excellent under all three conditions. For Rana cascadea and Bufo boreas, however, the UV-B blocking filter raised the percentage of eggs hatched from about 60% to close to 80%.

Figure 3.6

Hatching success rates in three amphibian species in the field. At each of two sites, eggs were placed in enclosures that were unshielded, shielded with an acetate screen that admitted UV-B radiation, or shielded with a Mylar^™ screen that blocked (more...)

The environmental programs of experimental embryology were a major part of the discipline when Entwicklungsmechanik was first established. However, it soon became obvious that experimental variables could be better controlled in the laboratory than in the field, and that a scientist could do many more experiments in the laboratory. Thus, field experimentation in embryology dwindled in the first decades of the twentieth century (see Nyhart 1995). However, with our increasing concern about the environment, this area of developmental biology has become increasingly important. Other recent work in this field will be detailed in Chapter 21.