The Questions of Developmental Biology

The males, however, divide their 10 chromosomes to produce some sperm with a haploid
number of 4 and other sperm with a haploid number of 6. The sperm with 4 chromosomes
degenerate. The sperm with 6 chromosomes fertilize the eggs with their 6 chromosomes to restore
the diploid chromosome number 12. These eggs overwinter, and when they hatch in the spring,
females emerge.
Morgan solved one riddle. The riddle of how the autumn weather regulates whether the
female reproduces sexually or parthenogenetically, however, remains unsolved. Similarly, we do
not know what regulates whether the diploid oocyte gives rise to male- or female-producing eggs.
Moreover, the same environmental factors are used differently by other aphid species. Figure 21.4
shows another type of life cycle found in aphids, involving an alternation of sexual and asexual
generations.
In Megoura viciae, temperature determines the sex early in development (with extreme
temperatures favoring the production of females). In female development, day length and
temperature determine whether the female will reproduce sexually or parthenogenetically, and a
combination of temperature and population density determine whether she will be winged or
wingless (Beck 1980). It appears that juvenile hormone controls the parthenogenetic/sexual
switch (the addition of JH to adults producing sexual offspring causes them to have
parthenogenetic offspring) and inhibits the formation of wings (Hardie 1981; Hardie and Lees
1985). But it is not known how the environmental changes become transformed into titers of JH,
or how the autumn weather (or perhaps declining hours of sunlight) causes the differential
movement of chromosomes into the polar body.