The Questions of Developmental Biology

quite well with all of this "hitchhiking" DNA. Animals also have a significant amount of foreign
DNA, but aneuploidy and polyploidy can be developmentally harmful to them. When plants are
aneuploid or polyploid, the consequences can be adaptive. Many flowers found in the florist shop
and the wheat used for bread flour are examples of successful polyploids.
Despite these major differences among many plants and animals, developmental genetic
studies are revealing some commonalities between them in the regulation of basic molecular
mechanisms of patterning, along with evolutionarily distinct solutions to the problem of creating
three-dimensional form from a single cell.
*The term plant loosely encompasses many organisms, from algae to flowering plants (angiosperms). Recent
phylogenetic studies show a common lineage for all green plants, distinct from the red and brown plants. This new
phylogenetic tree differs from older classification schemes in which the kingdom Plantae consisted of multicellular,
photosynthetic plants that develop from embryos protected by tissues of the parent plant. While this chapter focuses
primarily on the flowering plants, their developmental strategies are best understood in an evolutionary context.
Plant Life Cycles
The plant life cycle alternates between haploid and diploid generations. Embryonic development
is seen only in the diploid generation. The embryo, however, is produced by the fusion of
gametes, which are formed only by the haploid generation. So understanding the relationship
between the two generations is important in the study of plant development.
Unlike animals(see Chapter 2), plants have multicellular haploid and multicellular diploid stages
in their life cycle. Gametes develop in the multicellular haploid gametophyte (from the Greek
phyton, "plant"). Fertilization gives rise to a multicellular diploid sporophyte, which produces
haploid spores via meiosis. This type of life cycle is called a haplodiplontic life cycle (Figure
20.1). It differs from our own diplontic life cycle, in which only the gametes are in the haploid
state. In haplodiplontic life cycles, gametes are not the direct result of a meiotic division. Diploid
sporophyte cells undergo meiosis to produce haploid spores. Each spore goes through mitotic
divisions to yield a multicellular, haploid gametophyte. Mitotic divisions within the gametophyte
are required to produce the gametes. The diploid sporophyte results from the fusion of two
gametes. Among the Plantae, the gametophytes and sporophytes of a species have distinct
morphologies (in some algae they look alike). How a single genome can be used to create two
unique morphologies is an intriguing puzzle.
All plants alternate generations. There is an evolutionary trend from sporophytes that are
nutritionally dependent on autotrophic (self-feeding) gametophytes to the opposite -gametophytes
that are dependent on autotrophic sporophytes. This trend is exemplified by comparing the life
cycles of a moss, a fern, and an angiosperm (see Figures 20.2 20.4). (Gymnosperm life cycles
bear many similarities to those of angiosperms; the distinctions will be explored in the context of
angiosperm development.)
The "leafy" moss you walk on in the woods is the gametophyte generation of that plant (Figure
20.2). Mosses are heterosporous, which means they make two distinct types of spores; these
develop into male and female gametophytes. Male gametophytes develop reproductive structures
called antheridia (singular, antheridium) that produce sperm by mitosis. Female gametophytes
develop archegonia (singular, archegonium) that produce eggs by mitosis. Sperm travel to a
neighboring plant via a water droplet, are chemically attracted to the entrance of the archegonium,
and fertilization results.* The embryonic sporophyte develops within the archegonium, and the
mature sporophyte stays attached to the gametophyte. The sporophyte is not photosynthetic. Thus