Annalee Newitz in io9:
Dawkins was in no way the first person to think about culture in the context of tiny units of meaning that replicate themselves. Linguists had pondered this idea for over a century before Dawkins copied their memes. And in 1970, a few years before The Selfish Gene came out, a philosopher named Roland Barthes published a book called S/Z where he explored the idea of the “seme,” or a single unit of semantic meaning. Like Dawkins's meme, Barthes's seme could be a word, a song, or an image. A seme means many things at once; it is inherently unstable, what Barthes calls “a flicker of meaning.” For example, a description of a large house (the seme) can mean “wealth” or “loneliness” or “family.”
If you analyzed Limecat as a seme, you could say that it means “I am humiliated,” “You suck,” “This is awkward,” “I am all-powerful and regard you as ridiculous,” and any number of other possible things. Today's lolcats and animated gifs fit the definition of Barthes's seme as much as Dawkins's meme because they are used in so many situations to mean so many different things. They become a flicker of meaning in our internet conversations, an ambiguous rejoinder to a comment or a vague representation of a feeling.
Barthes's book S/Z offers, in part, a seme-by-seme analysis of a short story by Honoré de Balzac called “Sarassine.” He uses the story to demonstrate how semes are inherently unstable, taking on meanings and discarding them. This is crucial to Barthes's whole view of how narrative works. Like many philosophers of his time, Barthes insisted that cultural texts — whether books or sporting events — always have many meanings. This is partly because textual meaning is in the mind of the reader, and it's partly because language itself works by implication and suggestion. There is, in other words, no way to test a book in the lab and find out what its absolute meaning is.
If the meme is the basic unit of culture, I suppose you could say the seme is the basic unit of cultural ambiguity. The seme explains why memes never survive intact.