A Parasite in the Family

In the previous post, I wrote about an epiphytic red alga, and mentioned in passing the interesting phenomenon of adelphoparasitism, where a parasite is very closely related phylogenetically to its host. Since then, I've been wondering how such a situation arose, and specifically whether there was a connection between red algal adelphoparasitism and the complexities of red algal life cycles (see here for a summary).

Red algae fall into three to seven classes - Rhodellophyceae (which Yoon et al., 2006, divide into five, but as I haven't yet seen the paper I'll let it slide), Bangiophyceae and Florideophyceae. Rhodellophyceae are unicellular, and I confess I don't know the details of their life cycles. Bangiophyceae (which include Porphyra, the nori used in making sushi) alternate between distinct haploid and diploid generations. Florideophyceae include the vast majority of red algae, and verge on the completely insane in life style complexity. The basic florideophycean life cycle (which, as shown in the previous post, not all members of the class go through) involves no less than three alternating generations (I linked to the diagram here in the previous post, but I'll do it again because it's a good'un). Starting with the diploid tetrasporophyte, the tetrasporophyte releases haploid spores that settle and grow into gametophytes. Male gametophytes release spermatia (aflagellate sperm) that are captured by the female gametophytes and fertilise the carpogonia. The carpogonium (and this is the interesting part for this post) then grows into a carposporophyte, which remains attached to the parent gametophyte, releasing diploid spores that grow into new tetrasporophytes. So in effect, parasitism is already part of the florideophycean life cycle. Is it somehow possible that this parasitism is behind the rise of adelphoparasitism?

It's worth noting here that similar patterns to "adelphoparasitism" are not unique to red algae. They have also been recorded among social Hymenoptera as well as mistletoes. Red algal parasites have traditionally been divided between adelphoparasites (which are closely related to their hosts) and alloparasites (not so closely related). The two classes are also supposedly distinguished by the mode of parasitism. In both, after the parasite rhizoid invades the host it adheres to and fuses with the host cells, injecting parasite nuclei and mitochondria. In adelphoparasites, the parasite nuclei then multiply within the host cell, hijacking it and causing the formation of growths which release spores of the parasite species (Goff et al., 1997), In alloparasites, the parasite nuclei do not divide in the host cytoplasm, though they do alter its physiology to facilitate the transfer of nutrients from host to parasite, and (I assume) the parasite reproductive bodies grow from the parasite rhizoid itself. Goff et al. (1997) demonstrated that one 'genus' of adelphoparasites had actually arisen polyphyletically from the host 'genus'. Zuccarello et al. (2004) demonstrated the same thing for a 'family' of alloparasites. The latter authors therefore suggested that the terms 'adelphoparasite' and 'alloparasite' were not useful. However, this does still leave the question of the different cytoplasmic interactions (Zuccarello et al. implied that this might be due to the taxa studied belonging to different orders).

Goff et al. (1997) give two possible scenarios for the origin of parasitic red algae. In one, the parasites are ancestrally epiphytic, later becoming endophytic and eventually parasitic. In the second, the parasites derive directly from spores that lose the ability to survive independently of the parent. The existence of the carposporophyte, in my opinion, gives a lot of support to this option. One possibility is that adelphoparasites arose by the second method while alloparasites arose by the first.

Goff et al. also examined the main complaint towards the second origin - even if some mutant parasitic individual does arise, what is to stop it backcrossing to the parent population? How does the parasite become established as a new species? At present, there is no really satisfying answer to this question. Goff et al. do point out that parasitic taxa have life cycles taking a fraction of the time of the host species. At any given time, only a small percentage of the individuals in a population of algae are reproductive - perhaps the difference in timing of life cycles simply meant that the chance of backcrossing between parasite and non-parasite was too low to prevent speciation?

REFERENCES

Goff, L. J., J. Ashen & D. Moon. 1997. The evolution of parasites from their hosts: a case study in the parasitic red algae. Evolution 51 (4): 1068-1078.

Zuccarello, G. C., D. Moon & L. J. Goff. 2004. A phylogenetic study of parasitic genera placed in the family Choreocolacaceae (Rhodophyta). Journal of Phycology 40: 937-945.