Why are equally sized gametes so rare? The instability of isogamy and the cost of anisogamy. Hiroyuki Matsuda 1 and Peter A.
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Isogamy is a reproductive system where all gametes are morphologically similar, especially in terms of size. Its importance goes beyond specific cases: to this day non-anisogamous systems are common outside of multicellular animals and plants, they can be found in all eukaryotic super-groups, and anisogamous organisms appear to have isogamous ancestors. Furthermore, because maleness is synonymous with the production of small gametes, an explanation for the initial origin of males and females is synonymous with understanding the transition from isogamy to anisogamy.
Studies in green algae have generated some surprising new findings on the genetic changes that might have underpinned the evolution of male and female gametes. While prior studies in both animals and plants suggest that large-scale genetic expansion and differentiation must have occurred to enable the divergence of male and female chromosomes, new research in two closely related multicellular green algae suggests that the genetic differences may, in fact, have been far less dramatic. The researchers, led by Hisayoshi Nozaki, Ph.
The clear dominance of two-gender sex in recent species is a notorious puzzle of evolutionary theory. It has at least two layers: besides the most fundamental and challenging question why sex exists at all, the other part of the problem is equally perplexing but much less studied. Why do most sexual organisms use a binary mating system?
Available for iPhone or iPad:. Available for Mac and Windows Desktops and laptops:. Transition from isogamy to anisogamy, hence males and females, leads to sexual selection, sexual conflict, sexual dimorphism, and sex roles.
Skip to search form Skip to main content. Evolutionary trajectories explain the diversified evolution of isogamy and anisogamy in marine green algae. The evolution of anisogamy the production of gametes of different size is the first step in the establishment of sexual dimorphism, and it is a fundamental phenomenon underlying sexual selection.
This thesis develops theories for the evolution of alternative reproductive morphs, in the process exploring the evolutionary consequences of sperm limitation and developing an alternative theory for the evolution of the sexes and sex roles. Previous studies of the evolutionary consequences of sperm limitation have been restricted to morphological and phenological changes. I examine the potential of selection for increased gamete density to cause the evolution of alternative reproductive morphslarge eggs and small sperm, specialized male and female individuals and independent haploid and diploid stages in a lifecycle.
The molecular bases for the evolution of male—female sexual dimorphism are possible to study in volvocine algae because they encompass the entire range of reproductive morphologies from isogamy to oogamy. InCharlesworth suggested the model of a gamete size gene becoming linked to the sex-determining or mating type locus MT as a mechanism for the evolution of anisogamy. These data demonstrate very little sex-linked sequence divergence of MAT3 between the two sexes in other volvocine groups, though linkage between MAT3 and the mating locus appears to be conserved.
Sexual Reproduction in Animals and Plants pp Cite as. Colonial Volvocales green algae are a model lineage for the study of the evolution of sexual reproduction because isogamy, anisogamy, and oogamy are recognized within the closely related group, and several mating type sex -specific genes were identified in the closely related unicellular Chlamydomonas reinhardtii during the past century. Thus, it was speculated that the derived or minus mating type of C.