All this talk of algae life-cycles alternating between diploid and haploid stages takes me back to winter afternoons in undergraduate botany practical classes when the basics of plant life cycles were driven into us. Roughly speaking, the principles of biology can be split into those that are common sense and those that need to be explained over and over again to overcome our natural preconceptions. “Common sense”, in this instance, means that the principle in question roughly aligns to our own anthropogenic outlook. And one thing that humans – along with all other animals – don’t have is alternating generations.
A step backwards: “diploid” means a cell with a full complement of chromosomes and “haploid” is a cell with half a set of chromosomes (see note). In the human, the “haploid” phase is the short period of the life cycle between production of sperm and egg cells (collectively termed “gametes”) and the successful fertilisation of an egg by a sperm cell. So the human life-cycle consists solely of the alteration between a multicellular diploid phase (that’s you) and a unicellular haploid phase. In the case of all but the very simplest algae, there is a multicellular haploid phase (the “gametophyte”) which produces the gametes. These then fuse to produce a multicellular diploid phase (the “sporophyte”), some cells of which undergo meiosis to produce haploid spores. These spores germinate to form the next generation of gametophytes.
Got that? Probably not. It is not an easy concept to grasp partly because it does not accord with the human experience. Moreover, in flowering plants, the sporophyte is dominant and the gametophyte stages are reduced to “parts” of the flower (the entire male gametophyte, for example, is contained within the pollen grain). It does not help the cause that the most conspicuous group of plants have their alternation of generations largely hidden from view. When I was an undergraduate, knowing about alternation of generations was regarded as necessary if we were to understand the evolution of plants. Unfortunately, the experience of having plant life cycles drilled into them probably turned many undergraduates off botany forever. I am fairly sure that few UK university departments teach these principles in anything like the detail that my generation had to endure.
Alternation of generations isn’t something that the average biologist needs to know in great detail and, to be honest, I forgot a lot of the details. I had to refresh myself when I taught courses on algae and the lower plants to undergraduates in Nigeria and, once again, when I started boning up for the post about Audouinella. As Bill Farnham pointed out, I may not have got all the details right. No excuses for my own failings but this is yet another manifestation of a problem that I have mentioned before: that science has a “trailing edge” of knowledge as well as a “leading edge” (see “An inordinate fondness for … algae”). There is an enormous amount of information on algal life cycles in the literature but it is not always easy to find, especially when accessing the literature via internet search engines. We might not need the information very often but it is as easy to lose familiarity with the terminology as it is for an athlete to fall out of condition through lack of exercise.
Note: another way of looking at it is to say that the haploid phase has a full set of chromosomes whilst the diploid phase has two full sets. I was being deliberately anthropocentric in describing the haploid phase in this way. And, if we are going to be pedantic, every “multicellular diploid phase” necessarily starts out, albeit briefly, as a single celled diploid phase.