CHAPTER 16

During the process of development, cells commit to specific cell fates or in other words limit their capacity to differentiate into only specific cell types. Cells capable of differentiating into any other cell types are referred to as totipotent. During development, cells operate under positional directions incorporated in chemical gradients called the developmental field.

Sex determination is a developmental process functions via different genetic control systems in different taxa. In Drosophila, gender is determined by the ratio of X chromosomes to complete sets of autosomal chromosomes. Each set of autosomal chromosomes is usually symbolized as A (a normal diploid would consequently be AA). If the ratio of X to A is one the individual is female, if the ratio of X to A is 0.5 the individual is phenotypically male. If the ratio is intermediate the individual is classified as intersex. Note that the Y chromosome plays no role in determining the phenotypic sex in Drosophila. Individuals that are XAA are however sterile males. In mammals, phenotypic sex is determined solely through the presence (males) or absence (females) of the Y chromosome (Table 16-1).

The regulatory switch for gender determination in Drosophila is the sex lethal gene (sxl). Genes found influencing gender on the X chromosome are called numerator (NUM) genes and those on A are called denominator (DEN) genes. These genes encode transcription factors having basic helix-loop-helix motifs (basic helix-loop-helix proteins (bHLH)). The functional bHLH protein is a dimer, which spontaneously assembles if NUM-NUM monomers come together. However, if NUM-DEN or DEN-DEN dimers form the transcription factor is non-functional. The former occurs in XXAA individuals the latter in XYAA individuals. Once the sxl gene is turned on early in development, it produces the SXL protein that recognizes mRNA produced by RNA polymerase recognizing another promoter the late promoter. This mRNA is attached to by the early SXL protein that causes it to be spliced such that a functional late SXL protein is produced by post transcriptional processing. This leads to a female phenotype. Without early SXL, the late mRNA is spliced such that a nonfunctional protein is produced, and the individual becomes male.

In mammals sex determination centers on whether or not a testes is present. Two months into gestation, primordial germ cells migrate to the genital ridge atop the rudimentary kidney. If the germ cells have a Y chromosome, they cause testes formation and the Leydig Cells of the testes secrete testosterone which is a steroid hormone that can cross cell membranes and interact with steroid hormone receptors. The Steroid/androgen receptor complex acts as a Transcription Factor turning on the male genes.

The cytoskeleton is influential in early development because components such as microfilaments and microtubules have polarities (+ and -). The cytoskeleton acts as a guide for motor proteins to attach to and to transport cellular components from a + to - direction. Microfilaments (actin) are used in the zygote (and its descendents) of the nematode C. elegans to direct and cause the continually accumulation Polar granules toward the posterior portion of the cells prior to cytokinesis. This eventually leads to the germ line being generated by cells containing large quantities of P particles. This establishes one of the major fate decisions that occurs during development, namely whether cells will be part of the soma or the germ line.

An analogous process occurs in Drosophila except that microtubules serve to direct movement. Early development in Drosophila is somewhat unusual in that the mitotic divisions (the first 13) are not initially accompanied by cytokinesis. The cell therefore becomes multinucleated and is referred to as a syncidium. Following the ninth mitotic division the cell membrane at the posterior portion of the syncidium evaginates and pinches off cells high in polar granule concentration. These become the germ line cells Figure 16-14.

Messenger RNA can bind to either the + or - end of these cytoskeletal elements via the 3' Untranslated Regions of the mRNA (3'UTRs). This plays a major role in establishing the anterior-posterior (A-P) axis of the embryo. The concentration of two gene products BCD (from the bicoid gene) and HB-M (from the hunchback gene), both of which are transcription factors, are significant in establishing the A-P orientation (Figure 16-15). In the syncidium, the maternally produced mRNA for BCD is tethered to the - end of microtubules (the anterior side) and this increases BCD concentration in nearby nuclei. The more gradual A-P gradient for HB-M (another protein translated from maternally transcribed mRNA) is generated due to action of another protein NOS. Maternally derived HB-M mRNA is uniformly distributed within the syncidium, but is inactivated if bound to NOS. NOS mRNA binds to the + end of microtubules, so is in higher concentration at the posterior end of the syncidium, as will be its translated product NOS.

The dorsal ventral (DV)axis of Drosophila is generated due to a gradient in the concentration of a transcription factor, DL, coded for by the dorsal, dsl, gene. DL exists in an inactive form in the cytoplasm if bound to the CACT protein, or is an active transcription factor if released in the nucleus. Follicle cells on the ventral side of the oocyte secrete SPZ ligands that bind to TOLL transmembrane receptors. The signal transduction system causes DL to be phosphorylated and released from CACT, This occurs preferentially at higher concentrations of the ligand (as occur on the ventral side). Figures 16-18, 16-20.

The previous examples show that positional information can be generated by localization of mRNA within a cell (e.g. A-V orientation) or due to extracellular diffusible molecules- morphogens (e.g., D-V orientation). These patterns have generated gradients in transcription factors that influence zygotically expressed genes. Such genes, expressed in response to maternally supplied positional information are called cardinal genes. A-P cardinal genes are also called gap genes because mutant flies lack segments along their body plan. They are expressed due to different concentration domains in the A-P translation factors; these regions serve as developmental fields that induce cells within different domains to have different fates.

Gap gene products are transcription factors that regulate more fine scaled regulation. This regulation establishes both the number of segments and the segment identity. Gap genes in Drosophila subdivide the A-P axis into four domains. These genes influence the transcription of pair-rule genes that produce transcription factors that influence transcription in adjacent pairs of segments. Polarity is established within segments by protein products of segment-polarity genes that are activated by complexes of pair-rule gene products.

Segment identity is established by homeotic gene complexes. Two homeotic gene clusters exist in Drosophila: Antennapedia (ANT-C responsible for the head and anterior thorax) and Bithorax (BX-C responsible for posterior thorax and abdomen). Segment identity is established by the gap domains established by gap gene proteins activating homeotic genes in series of overlapping homeotic domains.

Mammals show analogous patterns in the organization of their homeotic genes within Hox clusters (4). Figure 16-33.

Similar developmental control systems exist in different animal taxa (Figure 19-21).