裸子植物起源于中泥盆纪,经历了漫长的进化历史,有物种形成和灭绝的脉冲,这导致了四个形态上不同的现存组,即,苏铁,银杏,针叶树和gnetophytes。一个多世纪以来,现存裸子植物内部的系统关系一直存在争议,因为不同的作者强调不同的性格。最近对裸子植物的系统基因组学研究给出了一致的拓扑结构,与现存的裸子植物分类为三类,五个子类,八个命令,13个家庭。这里,我们回顾了裸子植物系统学的历史观点,并特别参考了几个有问题的分类单元,并重新考虑了分类学家先前在系统学背景下强调的一些关键形态特征的演变。我们得出的结论是(1)苏铁包含两个家族,即,苏铁科和扎米科;(2)银杏是苏铁的姐妹,但不是针叶树,银杏和针叶树之间的相似性是平行进化的结果,包括单足生长模式,长芽中的pycnoxylic木材,和复合女性视锥细胞,银杏和苏铁之间的生殖相似性要么是突触的,要么是多形性的,例如,船形花粉,分支的花粉管,和有鞭毛的精子;(3)针叶树是近生的,其中嵌套有gnetophytes,因此,侏儒是衍生的针叶树,我们新界定的针叶树相当于Pinopsida,包括三个亚类,即,松科,奈迪科,和Cupressidae;(4)针叶树的肉质锥体多次起源,罗汉果科是南洋菜科的姐妹,三尖科和红豆杉科包括一个小进化枝,是柏科的姐妹;(5)三叶草科与红豆杉科不同,因为前一个家族拥有典型的雌性视锥细胞,种子的肉质部分来自于外皮的肉质,而后者家族减少了雌性视锥细胞,并且没有保留种子规模复合体的痕迹。
Gymnosperms originated in the Middle Devonian and have experienced a long evolutionary history with pulses of speciation and extinction, which resulted in the four morphologically distinct extant groups, i.e., cycads, Ginkgo, conifers and gnetophytes. For over a century, the systematic relationships within the extant gymnosperms have been debated because different authors emphasized different characters. Recent phylogenomic studies of gymnosperms have given a consistent topology, which aligns well with extant gymnosperms classified into three classes, five subclasses, eight orders, and 13 families. Here, we review the historical opinions of systematics of gymnosperms with special reference to several problematic taxa and reconsider the evolution of some key morphological characters previously emphasized by taxonomists within a phylogenomic context. We conclude that (1) cycads contain two families, i.e., the Cycadaceae and the Zamiaceae; (2) Ginkgo is sister to cycads but not to conifers, with the similarities between Ginkgo and conifers being the result of parallel evolution including a monopodial growth pattern, pycnoxylic wood in long shoots, and the compound female cones, and the reproductive similarities between Ginkgo and cycads are either synapomorphic or plesiomorphic, e.g., the boat-shaped pollen, the branched pollen tube, and the flagellate sperms; (3) conifers are paraphyletic with gnetophytes nested within them, thus gnetophytes are derived conifers, and our newly delimited coniferophytes are equivalent to the Pinopsida and include three subclasses, i.e., Pinidae, Gnetidae, and Cupressidae; (4) fleshy cones of conifers originated multiple times, the Podocarpaceae are sister to the Araucariaceae, the Cephalotaxaceae and the Taxaceae comprise a small clade, which is sister to the Cupressaceae; (5) the Cephalotaxaceae are distinct from the Taxaceae, because the former family possesses typical female cones and the fleshy part of the seed is derived from the fleshiness of integument, while the latter family has reduced female cones and preserves no traces of the seed scale complexes.