The term "C-value enigma" represents an update of the more common but outdated term "C-value paradox" (Thomas 1971), being ultimately derived from the term "C-value" (Swift 1950) in reference to haploid nuclear DNA contents.
In barley, haploids can be produced by wide crossing with the related species Hordeum bulbosum, fertilization is effected, but during the early stages of seed development the H. bulbosum chromosomes are eliminated leaving a haploid embryo.
The monogamy hypothesis, formulated by Jacobus Boomsma, is currently the leading hypothesis concerning the evolution of eusociality and uses Hamilton's kin selection approach in a way that applies to both haploid and diploid organisms.
The foraminiferal life-cycle involves an alternation between haploid and diploid generations, although they are mostly similar in form.
The role of karyogamy in sexual reproduction can be demonstrated most simply by single-celled haploid organisms such as the algae of genus Chlamydomonas or the yeast Saccharomyces cerevisiae.