lpetrich

Many of us have seen some interesting examples of deep homology across the animal kingdom:

• The famous Hox genes, which control front-to-rear patterning. Their order in the genome even corresponds to their front-to-rear expression locations.
• Dorsovental-patterning genes, including their reverse interpretation between arthropods and vertebrates (a fly's back is homologous to a frog's belly, and a fly's belly is homologous to a frog's back).
• Limb-growth control; genes similar to distal-less are expressed at the ends of limbs across the animal kingdom -- limbs that do not seem anything like homologous! (G. Panganiban, et al., Proc. Natl. Acad. Sci. U.S.A. 94, 5162 (1997))
• Early development of various organs, like the central nervous system, the heart (tinman and the like), and eyes (Pax-6). There is even a <a href="http://www.amnh.org/naturalhistory/features/0400_feature1.html" target="_blank">heart-throat homology</a>, making a hearbeat homologous to a gulp.
• Sleep mechanisms (from an article in <a href="http://gnn.tigr.org" target="_blank">the Genome News Network</a>).

Here are two more:

• Germline segregation: cells that become sperm and egg cells get separated out early in development.
• Apoptosis or Programmed Cell Death, what I like to call cellular hara-kiri.

Germline segregation is a common feature in the animal kingdom; those cells that will produce egg and sperm cells get distinguished from other cells early in embryonic development. Here is <a href="http://www.uib.no/fa/sars/group4research.htm" target="_blank">some work</a>, on some widely-shared germline-development genes; several are shared between fruit flies and nematodes, and one of them has been found in fish and chickens. Also, here are <a href="http://hoya.bio.titech.ac.jp/mhoshi/projects/kobayashi-e.html" target="_blank">some summaries</a> of various work on this subject, including discovery of a homology of some germline-development mechanisms between fruit flies and frogs, at least as far as I could tell from the awkward English.

Here is <a href="http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/A/Apoptosis.html" target="_blank">an introduction</a> to cellular hara-kiri. This mechanism is triggered by a variety of causes, like being made unnecessary during continued growth and having defective genetic material. A cell that fails to detect such defects can become a cancer cell. This mechanism is widely conserved, though the triggering of it does vary; genes involved in it are known from fruit flies and nematodes as well as from vertebrates. It's been a bit difficult for me to find a comparison that does not get lost in the details of specific genes, such as some table listing which homologues have been found where.

And apoptosis is found outside of the animal kingdom; <a href="http://www.bspp.org.uk/icpp98/1.3/7S.html" target="_blank">plants also have that mechanism</a>, which is highly conserved in them, and even <a href="http://www.purdue.edu/UNS/html4ever/990902.Bressan.osmotin.html" target="_blank">fungi</a> do. Here's <a href="http://www.sciam.com/askexpert_question.cfm?articleID=00077E50-7270-1C72-9EB7809EC588F2D7&catID=3&topicID=3" target="_blank">another short article on this subject</a>. It is not clear how conserved this mechanism is among animals, plants, and fungi. Was it invented only once and then inherited? Or was it invented once among early animals, once among early (multicelled) plants, and once or more among fungi?