School of Biological Sciences, University of Adelaide, Adelaide, South Australia, Australia
School of Biological Sciences, University of Adelaide, Adelaide, South Australia, Australia.
Proc Biol Sci. 2018 Nov 7;285(1890):20181954. doi: 10.1098/rspb.2018.1954.
Although the gamete competition theory remains the dominant explanation for the evolution of anisogamy, well-known exceptions to its predictions have raised doubts about the completeness of the theory. One of these exceptions is isogamy in large or complex species of green algae. Here, we show that this exception may be explained in a manner consistent with a game-theoretic extension of the original theory: a constraint on the minimum size of a gamete may prevent the evolution of continuously stable anisogamy. We show that in the volvocine algae, both gametes of isogamous species retain an intact chloroplast, whereas the chloroplast of the microgamete in anisogamous species is invariably degenerate. The chloroplast, which functions in photosynthesis and starch storage, may be necessary to provision a gamete for an extended period when gamete encounter rates are low. The single chloroplast accounts for most of the volume of a typical gamete, and thus may constrain the minimum size of a gamete, preventing the evolution of anisogamy. A prediction from this hypothesis, that isogametes should be larger than the microgametes of similar-size species, is confirmed for the volvocine algae. Our results support the gamete competition theory.
虽然配子竞争理论仍然是解释雌雄配子异形性进化的主要理论，但该理论的一些明显例外引起了人们对其完整性的怀疑。其中一个例外是绿藻门大型或复杂物种中的同配生殖。在这里，我们表明，这种例外可以用对原始理论的博弈论扩展来解释：配子最小尺寸的限制可能会阻止连续稳定的雌雄配子异形性的进化。我们表明，在绿藻门中，同配生殖物种的两个配子都保留了完整的叶绿体，而异形配子的微配子的叶绿体则总是退化的。叶绿体在光合作用和淀粉储存中起作用，当配子相遇率低时，它可能为配子提供延长的营养期所必需。单个叶绿体占典型配子体积的大部分，因此可能限制了配子的最小尺寸，从而阻止了雌雄配子异形性的进化。这一假说的一个预测是，同配生殖的配子应该比类似大小物种的微配子更大，这一预测在绿藻门中得到了证实。我们的结果支持了配子竞争理论。
