Friedman William E, Carmichael Jeffrey S
Department of Environmental, Population and Organismic Biology, University of Colorado, Boulder, Colorado, 80309.
Department of Biology, University of North Dakota, Grand Forks, North Dakota, 58202.
Evolution. 1998 Aug;52(4):1016-1030. doi: 10.1111/j.1558-5646.1998.tb01830.x.
Seed plant female gametophytes are focal points for the evolutionary modification of development. From a structural perspective, the most divergent female gametophytes among all seed plants are found in Gnetum, a clade within Gnetales. Coenocytic organization at sexual maturity, absence of defined egg cells (free nuclei are fertilized), lack of centripetal cellularization, and postfertilization development of embryo-nourishing tissues are features of the female gametophytes of Gnetum unparalleled among seed plants. Although the female gametophyte of Gnetum retains the three basic phases of somatic development common to female gametophytes of plesiomorphic seed plants (free nuclear development, cellularization, cellular growth), the timing of fertilization has been accelerated relative to the rate of somatic development. As a consequence, the female gametophyte of Gnetum matures sexually (is fertilized) at a juvenile (compared with the ancestral somatic ontogeny) and free nuclear stage of somatic development, thereby precluding differentiation of egg cells. Unlike progenetic animals, where truncation of somatic ontogeny evolves in tandem with acceleration in the timing of sexual maturation, the female gametophyte of Gnetum completes the entire ancestral somatic ontogeny after precocious sexual maturation. This results in the evolution of postfertilization development of embryo-nourishing female gametophyte tissues, a phenomenon unique among seed plants. Nonheterochronic developmental innovations have also played important roles in the evolution of the female gametophyte of Gnetum. Centripetal cellularization, which is always associated with the phase change from coenocytic to cellular organization among plesiomorphic seed plant female gametophytes, is lacking in Gnetum. Instead, during early phases of development, apomorphic free nuclear organization is coupled with a highly anomalous pattern of cellularization. Stage-specific innovations during early development in the female gametophyte of Gnetum do not affect plesiomorphic aspects of later phases of development. Thus, a complex array of heterochronic and nonheterochronic developmental innovations have played critical roles in the ontogenetic evolution of the highly apomorphic female gametophyte of Gnetum.
种子植物的雌配子体是发育进化修饰的焦点。从结构角度来看，所有种子植物中最具差异的雌配子体存在于买麻藤属植物中，买麻藤属是买麻藤目的一个分支。性成熟时的多核体组织、缺乏明确的卵细胞（游离核受精）、缺乏向心细胞化以及胚营养组织的受精后发育是买麻藤属植物雌配子体的特征，在种子植物中无与伦比。尽管买麻藤属植物的雌配子体保留了原始种子植物雌配子体体细胞发育的三个基本阶段（游离核发育、细胞化、细胞生长），但受精时间相对于体细胞发育速率有所加快。因此，买麻藤属植物的雌配子体在体细胞发育的幼年（与祖先体细胞个体发育相比）和游离核阶段就达到性成熟（受精），从而排除了卵细胞的分化。与幼态发育动物不同，在幼态发育动物中，体细胞个体发育的缩短与性成熟时间的加速同步进化，而买麻藤属植物的雌配子体在早熟性成熟后完成整个祖先体细胞个体发育。这导致了胚营养雌配子体组织受精后发育的进化，这在种子植物中是独特的现象。非异时性发育创新在买麻藤属植物雌配子体的进化中也发挥了重要作用。向心细胞化在原始种子植物雌配子体中总是与从多核体组织到细胞组织的相变相关联，而买麻藤属植物中则缺乏。相反，在发育早期阶段，特化的游离核组织与高度异常的细胞化模式相结合。买麻藤属植物雌配子体早期发育阶段的阶段特异性创新不会影响后期发育的原始特征。因此，一系列复杂的异时性和非异时性发育创新在买麻藤属植物高度特化的雌配子体个体发育进化中发挥了关键作用。
