Abstract：
The evolution of the land plant alternation of generations has been an open question for the past 150 years. Two hypotheses have dominated the discussion: the antithetic hypothesis, which posits that the diploid sporophyte generation arose de novo and gradually increased in complexity, and the homologous hypothesis, which holds that land plant ancestors had independently living sporophytes and haploid gametophytes of similar complexity. Changes in ploidy levels were unknown to early researchers. The antithetic hypothesis is contradicted by generation cycles in Lower Devonian Rhynie chert plants, whose sporophytes and gametophytes have similar morphologies and by some Silurian sporophytes whose complexity exceeds that of Rhynie chert sporophytes. The oldest unambiguous bryophyte gametophytes (thalli) are from the upper Middle Devonian, with an unconnected sporophyte nearby. Based on the 2024 discovery that conjugate algae are paraphyletic to land plants, we present a new hypothesis for the evolution of the land plant generation cycle, focusing on labile ploidy levels and types of reproduction found in conjugate algae. Our \'sexual lability\' hypothesis assumes a period of unstable generation cycles (as regards ploidy), likely with predominant clonal growth, as is common in conjugate algae, resulting in sporophytes and gametophytes of similar morphology. When sexual reproduction became stabilized, the timing of gamete fusion, meiosis, and resistant wall formation, which are heterochronic in some conjugate algae, became standardized, with wall formation permanently delayed. In our scenario, independently living adult sporophytes are the land plant ancestral condition, and life-long sporophyte retention on the gametophyte is a bryophyte apomorphy.
摘要：
在过去的150年中，陆生植物交替的演变一直是一个悬而未决的问题。两个假设主导了讨论：反对假设，假设二倍体孢子体的产生是从头产生的，并且复杂性逐渐增加，和同源假说，认为陆地植物祖先具有相似复杂性的独立生活的孢子体和单倍体配子体。倍性水平的变化是早期研究人员未知的。反对假说与下泥盆纪Rhyniechert植物的生成周期相矛盾，其孢子体和配子体具有相似的形态，并且某些志留纪孢子体的复杂性超过了Rhyniechert孢子体。最古老的明确苔藓植物配子体（thalli）来自中泥盆纪，附近有一个未连接的孢子体。基于2024年的发现，共轭藻类与陆地植物是共生的，我们为陆地植物生成周期的进化提出了一个新的假设，专注于共轭藻类中的不稳定倍性水平和繁殖类型。我们的“性不稳定性”假设假定了一段不稳定的世代周期（关于倍性），可能是主要的克隆生长，正如在共轭藻类中常见的那样，形成形态相似的孢子体和配子体。当有性生殖变得稳定时，配子融合的时机,减数分裂,和抵抗壁的形成，在一些共轭藻类中是异慢性的，变得标准化了，墙的形成永久延迟。在我们的场景中，独立生活的成年孢子体是陆地植物的祖先条件，而终生保留在配子体上的孢子体是苔藓植物的脱形。
