In some creatures, including humans, the move from one phase to the next has an obvious continuity. In other animals, things are more clearly punctuated
In some creatures, including humans, the move from one phase to the next has an obvious continuity. Fertilised eggs turn into fetuses, which become babies, who grow into two different sorts of adult, which, between them, can then produce new fertilised eggs. In other animals things are more clearly punctuated. The embryo growing inside a butterfly egg turns into a series of leaf-munching and moulting caterpillars, called instars.
The current belief is that these stem from the mixing of genes. By producing genetically novel individuals, sex poses a problem for parasites and pathogens and provides flexibility in changing circumstances. Those advantages compensate for its inefficiency., a nematode worm much favoured in biology laboratories, reproduces asexually in benign environments but starts creating some males when things get more challenging, in order to mix things up a bit.
After gastrulation, more and more cell lines have their future possibilities pared down as molecular switches disable some genes and promote the activity of others. Each cell line is thus guided along a path that leads to its specialisation as part of an organ. A few laggards, called stem cells, drop out of this journey. Their role is to generate, throughout an individual’s life, replacements for cells that have died.
Cells in gametophytes, the mating body type, have a single complement of chromosomes—a state known as being “haploid”, which is also seen in the eggs and sperm of animals. It is the process of creating haploid cells that sees half of each parent’s genome scrapped in sexual reproduction. Unlike eggs and sperm, though, these haploid cells can grow and differentiate themselves, creating the gametophyte body.
In flowering plants like oaks, progeny-dispersal is achieved not through haploid spores but instead by the fertilised embryo sacs creating embryo-containing seeds—which, in oaks, are called acorns. The embryo in an acorn lacks precursors of many adult organs. Leaves are grown later, as required, from stem cells known as meristems. But it is equipped with an incipient root and stem, and also has two food-storing leaves, called cotyledons.
For an adult to reproduce, and thus pass on its genes, it must first find a mate. Sometimes the finding is done directly by the sperm—fern’s sperm swim from the antheridia in which they form, through films of water, in search of the egg-bearing archegonia of other gametophytes. Sometimes it is done by the adults, through courtship rituals or competitions. Many flowering plants exploit a go-between in the form of a pollinating insect, bat or bird.
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