Repetitions and Duplications: double, triple and multiple
Say you’ve got “trout-au-bleu” (steamed/poached trout) on your plate and you dig in: no doubt you’ll notice the repetitive arrangement of the trunk musculature. Like folded serviettes, one like the other, the muscle segments (known as myomeres) are highly repetitive. Such repetitiveness, also called duplicity, is something one can find in numerous organisms involving numerous organs and not just the skeletal muscles (whose structural units, i.e. the so-called muscle fibres with their regular actin-myosin striation, are also highly repetitive). Series of identical muscle segments exist already in the lancet Amphioxus, a small filter-feeding creature basal to all animals with backbones.
Talking about backbones, how many vertebrae do a frog, a human, a blue whale and a snake have? The answer is 9, 33, 64 and at least 200, respectively. Here again, we see the principle of repetitiveness and its relation to body size and functionality. A short animal that moves around mostly by hopping would find additional vertebrae unnecessary, but the huge whale and the elongated limbless snakes need extra muscle packages (remember the myomeres above) and they, because of their attachment sites, usually depend on the number of vertebrae to support them.
Starfish are pentameric animals and are supposed to have 5 arms, but species like, for instance the sunflower starfish Pycnopodia helianthoides, can have 20 or slightly more. The ultimate of repetitiveness in relation to appendages are no doubt the centipedes and millipedes. Typical house and garden centipedes have maximally only 15 pairs of more or less identical legs (except for the last pair), but species belonging to the genus Scolopendra have 21-23 leg pairs and the thin and eyeless geophilomorph centipedes contain species with 221 and even slightly more pairs of legs. If that’s not ‘leggy’ enough, let’s take a look at the millipedes. Counting from the head the first seven body segments look like regular segments and those with legs possess one pair each. But from then on to the end of a millipede we notice so-called diplo-segments, with each one of them having 2 pairs of legs. The longest such millipedes beat the leggiest centipede “hands down” (better “feet down”), for Illacme plenipes, can have several hundred pairs of legs (and operate them in a backward and forward gear!).
Earthworms are a wonderful example of the principle of repetitiveness in animals, for their body segments, again with the exception of the segments in the front, are more or less identical externally and internally with nervous system ganglia and excretory organs in each one of them. That the so-called nephrons (with the Loop-of-Henle and descending and ascending branch), of which we have about 1 million in the kidney, are highly repetitive units is also noteworthy. And so is the homodontal set of teeth in dolphins, in which depending on the species up to 200 cone-shaped identical teeth can occur. The examples show that there are situations where it makes sense to have more of the same and gene or chromosomal duplications have frequently occurred and driven evolution. However, the opposite namely reductions and deletions are equally common and are also often seen in animals, especially when a smaller number of units provides a selective advantage (can you think of examples?). Well, that’ll be a topic for a future blog.
© Dr V.B. Meyer-Rochow and http://www.bioforthebiobuff.wordpress.com, 2019.
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