Bennici Andrea
Dipartimento di Biologia Vegetale, P.le delle Cascine 28, 50144 Florence, Italy.
Riv Biol. 2005 Sep-Dec;98(3):469-80.
The origin of the sporophyte in land plants represents a fundamental phase in the plant evolution. Today this subject is controversial and, in my opinion, scarcely considered in our textbooks and journals of botany, in spite of its importance. There are two conflicting theories concerning the origin of the alternating generations in land plants: the "antithetic" and the "homologous" theory. These have never been fully resolved. The antithetic theory maintains that the sporophyte and gametophyte generations are fundamentally dissimilar and that the sporophyte originated in an ancestor organism with haplontic cycle by the zygote dividing mitotically rather than meiotically, and with a developmental pattern not copying the developmental events of the gametophyte. The sporophyte generation was an innovation of critical significance for the land-plant evolution. By contrast, the homologous theory simply stated that a mass of cells forming mitotically from the zygote adopted the same developmental plan of the gametophyte, but giving origin to a diploid sporophyte. In this context, a very important question concerns the possible ancestor or ancestors of the land plants. Considerable evidences at morphological, cytological, ultrastructural, biochemical and, especially, molecular level, strongly suggest that the land plants or Embryophyta (both vascular and non-vascular) evolved from green algal ancestor(s), similar to those belonging to the genus Coleochaete, Chara and Nitella, living today. Their organism is haploid for most of their life cycle, and diploid only in the zygote phase (haplontic cycle). On the contrary, the land plants are characterized by a diplo-haplontic life cycle. Several questions are implied in these theories, and numerous problems remain to be solved, such as, for example, the morphological difference between gametophyte and sporophyte (heteromorphism, already present in the first land plants, the bryophytes), and the strong gap existing between these last with a sporophyte dependent on the gametophyte, and the pteridophytes having the gametophyte and sporophyte generations independent. On the ground of all of the evidences on the ancestors of the land plants, the antithetic theory is considered more plausible than the homologous theory. Unfortunately, no phylogenetic relationship exists between some green algae with diplontic life cycle and the land plants. Otherwise, perhaps, it should be possible to hypothesize another scenario in which to place the origin of the alternating generations of the land plants. In this case, could the gametophyte be formed by gametes produced from the sporophyte, through their mitoses or a delayed fertilization process?
陆生植物中孢子体的起源代表了植物进化中的一个基本阶段。如今，这个话题存在争议，而且在我看来，尽管其很重要，但在我们的植物学教科书和期刊中却很少被提及。关于陆生植物世代交替的起源有两种相互冲突的理论：“对立”理论和“同源”理论。这些理论从未得到完全解决。对立理论认为，孢子体世代和配子体世代本质上是不同的，孢子体起源于具有单倍体周期的祖先生物体，其合子进行有丝分裂而非减数分裂，并且发育模式不复制配子体的发育过程。孢子体世代对于陆生植物的进化具有至关重要的创新意义。相比之下，同源理论简单地指出，由合子通过有丝分裂形成的一团细胞采用了与配子体相同的发育计划，但产生了一个二倍体孢子体。在这种情况下，一个非常重要的问题涉及陆生植物可能的一个或多个祖先。在形态学、细胞学、超微结构、生物化学，尤其是分子水平上的大量证据有力地表明，陆生植物或胚植物（包括维管植物和非维管植物）是从绿色藻类祖先进化而来的，类似于现今属于鞘毛藻属、轮藻属和裸藻属的藻类。它们的生物体在其生命周期的大部分时间里是单倍体，仅在合子阶段是二倍体（单倍体周期）。相反，陆生植物的特征是具有双单倍体生命周期。这些理论中隐含了几个问题，并且仍有许多问题有待解决，例如，配子体和孢子体之间的形态差异（异形性，最早出现在第一批陆生植物苔藓植物中），以及最后这些植物与依赖配子体的孢子体之间存在的巨大差距，还有配子体和孢子体世代独立的蕨类植物。基于所有关于陆生植物祖先的证据，对立理论被认为比同源理论更具合理性。不幸的是，一些具有二倍体生命周期的绿藻与陆生植物之间不存在系统发育关系。否则，也许应该有可能假设另一种情况来解释陆生植物世代交替的起源。在这种情况下，配子体是否可以由孢子体产生的配子通过有丝分裂或延迟受精过程形成呢？
