7 1 Sexual Reproduction

0
29

Moreover, the discovery of frequent macroalgal cryptic speciation has not been accompanied by the study of the evolutionary ecology of those lineages, and, thus, an understanding of the mechanisms underlying such rampant speciation remain elusive. In this perspective, we aim to further the discussion and interest in species concepts and speciation processes in macroalgae. We propose a conceptual framework to enable phycological researchers and students alike to portray these processes in a manner consistent with dialogue at the forefront of evolutionary biology. We define a macroalgal species as an independently evolving metapopulation lineage, whereby we can test for reproductive isolation or the occupation of distinct adaptive zones, among other mechanisms, as secondary lines of supporting evidence. This ushered in the modern era of pteridophyte systematics, which has witnessed increasingly sophisticated analyses of the evolutionary relationships of seed-free vascular plants (Pryer et al. 2001;Qi et al. 2018;Rothfels et al. 2015;Schneider et al. 2004;Shen et al. 2018).

Subsequent adaption to the desiccating land environment, which makes sexual reproduction difficult, might have resulted in the simplification of the sexually active gametophyte, and elaboration of the sporophyte phase to better disperse the waterproof spores. The tissue of sporophytes and gametophytes of vascular plants such as Rhynia preserved in the Rhynie chert is of similar complexity, which is taken to support this hypothesis. By contrast, modern grenade mod vape vascular plants, with the exception of Psilotum, have heteromorphic sporophytes and gametophytes in which the gametophytes rarely have any vascular tissue. The third life-cycle type, employed by some algae and all plants, is called alternation of generations. These species have both haploid and diploid multicellular organisms as part of their life cycle. The haploid multicellular plants are called gametophytes because they produce gametes.

So while Siluro-Devonian plants such as Rhynia and Horneophyton possessed the physiological equivalent of roots, roots – defined as organs differentiated from stems – did not arrive until later. Unfortunately, roots are rarely preserved in the fossil record, and our understanding of their evolutionary origin is sparse. An endodermis may have evolved in the earliest plant roots during the Devonian, but the first fossil evidence for such a structure is Carboniferous. The endodermis in the roots surrounds the water transport tissue and regulates ion exchange between the groundwater and the tissues and prevents unwanted pathogens etc. from entering the water transport system. The endodermis can also provide an upwards pressure, forcing water out of the roots when transpiration is not enough of a driver.

These phenotypes have been selected by horticulturists for their increased number of petals. Several studies on diverse plants like petunia, tomato, Impatiens, maize, etc. have suggested that the enormous diversity of flowers is a result of small changes in genes controlling their development. It seems that on the level of the organ, the leaf may be the ancestor of the flower, or at least some floral organs. When some crucial genes involved in flower development are mutated, clusters of leaf-like structures arise in place of flowers. Thus, sometime in history, the developmental program leading to formation of a leaf must have been altered to generate a flower.

Despite their lack of greater physiological tolerances, gametophytes of several species occurred over a wider elevational range than conspecific sporophytes. Our results demonstrate that filmy fern gametophytes and sporophytes differ in their physiology and niche requirements, and point to the importance of microhabitat in shaping the evolution of water-use strategies in vascular plants. Partially clonality is an incredibly common reproductive mode found across all the major eukaryotic lineages.

Variance in ploidy levels between gametophytes and sporophytes results from differences in the basic condition of nuclei, which is haploid and diploid, respectively. It is possible that higher endopolyploidy in gametophytes could appear in evolution as compensation for haploidy . Several copies of genes with cumulative effects on the eventual phenotype lead to a higher number of products of a given gene and therefore to faster synthesis of proteins. This can contribute to faster growth, development, and metabolic processes , allowing the gametophyte to produce sufficient metabolites for nourishment of the sporophyte. Alternatively, endopolyploidy may simply compensate for low DNA content in gametophytic or sporophytic tissues and help to promote growth and development through nucleotypic effects. Recent studies of endopolyploidy in diploid and isogenic polyploid angiosperms have revealed that diploids tend to show higher endopolyploidy than autopolyploids , possibly due to compensation for lower basic inherited organism-specific ploidy.

Martins and co-worker investigated the thermal traits for gametogenesis, sexual reproduction and sporophyte recruitment of Arctic and North Sea Laminaria digitata. The experiments were conducted in the laboratory in a common garden set-up. However, the experimental temperature treatment was clearly biased towards the southern population; disregarding the fact that temperature adaptation and history of the two populations are different. This is contrary to the data presented and the conclusion was based on missed logical interpretation. Moreover, the suggestion that thermal characteristics of the two populations diverge over evolutionary time scales is speculative and not supported by the data presented.