3 years ago

No universal differences between female and male eukaryotes: anisogamy and asymmetrical female meiosis

Lindsay Jackson Derraugh, Jessica Carpinone, Root Gorelick
We previously showed that, across eukaryotes, universal differences do not exist between diploid females and males; hence, in the present study, we examine haploid stages. Unlike animal sperm, flowering plant sperm have nuclear pores, and so cannot be used to distinguish haploid females from males. Female and male gametes are not complementary: eggs and can fertilize eggs, whereas sperm can fertilize sperm, in some taxa. One sex of gametes is not universally parasitic on the other. Commonly held differences between eggs and sperm (e.g. only eggs, and not sperm, are large, long-lived, and immobile) do not apply to many eukaryotes. Many or all angiosperms have small eggs, sperm without flagella, and long-lived pollen. In many plants, we supposedly can distinguish females from males, although they have similar sized gametes. Theories of anisogamy are based on less energy being required to produce small sperm than large eggs. However any selective advantage of smaller sperm is nullified by sequestering most cytoplasmic biomass as residual bodies following meiosis in metazoa. Moreover, zoidogamous gymnosperms have numerous huge sperm. In both animals and plants, sperm are not the immediate products of meiosis but, instead, develop afterwards via haploid cell divisions. Consequently, we examined whether the most promising universal sexual difference in animals applies to plants and stramenopiles, namely asymmetrical female meiosis vs. symmetrical male meiosis. A few angiosperms (and some animals) have asymmetrical male meiosis, whereas many plants have symmetrical female meiosis. All bryophytes and stramenopiles with UV chromosomes do not have asymmetrical female meiosis insofar as meiosis produces two female and two male gametophytes. Homosporous monilophytes, lycophytes, and anthocerotophytes lack female vs. male meiosis, although they have distinct eggs and sperm. Therefore, currently, there are no universal criteria for distinguishing females from males across all animals, across all plants or across all stramenopiles, let alone across all eukaryotes.

Publisher URL: http://onlinelibrary.wiley.com/resolve/doi

DOI: 10.1111/bij.12874

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