Molecular Control of Sporophyte-Gametophyte Ontogeny and Transition in Plants

Alternation of generations between a sporophytic and gametophytic developmental stage is a feature common to all land plants. This review will discuss the evolutionary origins of these two developmental programs from unicellular eukaryotic progenitors establishing the ability to switch between haploid and diploid states. We will compare the various genetic factors that regulate this switch and highlight the mechanisms which are involved in maintaining the separation of sporophytic and gametophytic developmental programs. While haploid and diploid stages were morphologically similar at early evolutionary stages, largely different gametophyte and sporophyte developments prevail in land plants and finally allowed the development of pollen as the male gametes with specialized structures providing desiccation tolerance and allowing long-distance dispersal. Moreover, plant gametes can be reprogrammed to execute the sporophytic development prior to the formation of the diploid stage achieved with the fusion of gametes and thus initially maintain the haploid stage. Upon diploidization, doubled haploids can be generated which accelerate modern plant breeding as homozygous plants are obtained within one generation. Thus, knowledge of the major signaling pathways governing this dual ontogeny in land plants is not only required for basic research but also for biotechnological applications to develop novel breeding methods accelerating trait development.

Timing of meristem initiation and maintenance determines the morphology of fern gametophytes

The alternation of generations in land plants occurs between the sporophyte phase and the gametophyte phase. The sporophytes of seed plants develop self-maintained, multicellular meristems, and these meristems determine plant architecture. The gametophytes of seed plants lack meristems and are heterotrophic. In contrast, the gametophytes of seed-free vascular plants, including ferns, are autotrophic and free-living, developing meristems to sustain their independent growth and proliferation. Compared to meristems in the sporophytes of seed plants, the cellular mechanisms underlying meristem development in fern gametophytes remain largely unknown. Here, using confocal time-lapse live imaging and computational segmentation and quantification, we determined different patterns of cell divisions associated with the initiation and proliferation of two distinct types of meristems in fern gametophytes. Our results reveal how the simple timing of a switch between two meristems has considerable consequences for the divergent gametophyte morphologies of two closely related ferns from Pteridaceae (Pteris and Ceratopteris). Our result provides evolutionary insight into the function and regulation of gametophyte meristems in seed-free vascular plants.

Epigenetic reprogramming rewires transcription during the alternation of generations in Arabidopsis

Alternation between morphologically distinct haploid and diploid life forms is a defining feature of most plant and algal life cycles, yet the underlying molecular mechanisms that govern these transitions remain unclear. Here, we explore the dynamic relationship between chromatin accessibility and epigenetic modifications during life form transitions in Arabidopsis. The diploid-to-haploid life form transition is governed by the loss of H3K9me2 and DNA demethylation of transposon-associated cis-regulatory elements. This event is associated with dramatic changes in chromatin accessibility and transcriptional reprogramming. In contrast, the global loss of H3K27me3 in the haploid form shapes a chromatin accessibility landscape that is poised to re-initiate the transition back to diploid life after fertilization. Hence, distinct epigenetic reprogramming events rewire transcription through major reorganization of the regulatory epigenome to guide the alternation of generations in flowering plants.

Haemogregarines and Criteria for Identification

Apicomplexa is a phylum that includes all parasitic protozoa sharing unique ultrastructural features. Haemogregarines are sophisticated apicomplexan blood parasites with an obligatory heteroxenous life cycle and haplohomophasic alternation of generations. Haemogregarines are common blood parasites of fish, amphibians, lizards, snakes, turtles, tortoises, crocodilians, birds, and mammals. Haemogregarine ultrastructure has been so far examined only for stages from the vertebrate host. PCR-based assays and the sequencing of the 18S rRNA gene are helpful methods to further characterize this parasite group. The proper classification for the haemogregarine complex is available with the criteria of generic and unique diagnosis of these parasites.

Plant Sexual Reproduction: Perhaps the current plant two-sex model should be replaced with three- and four-sex models?

The current plant two-sex model makes the assumption that there are only two sexual reproductive states: male and female. However, the application of this model to the plant alternation of generations requires the subtle redefinition of several common terms related to sexual reproduction, which also seems to obscure aspects of one or the other plant generation: For instance, the homosporous sporophytic plant is treated as being “asexual,” and the gametophytes of angiosperms treated like mere gametes. In contrast, the proposal is made that the sporophytes of homosporous plants are indeed sexual reproductive organisms, as are the gametophytes of heterosporous plants. This view requires the expansion of the number of sexual reproductive states we accept for plants, therefore a three-sex model for homosporous plants and a four-sex model for heterosporous plants are described and then contrasted with the current two-sex model. These new models allow the use of sexual reproductive terms in a manner largely similar to that seen in animals, and may better accommodate the plant alternation of generations life cycle than does the current plant two-sex model. These new three-sex and four-sex models may also help stimulate new lines of research, and examples of how they might alter our view of the flower, and may lead to new perspectives in terms of sexual determination, are presented. Thus it is suggested that plants have more than merely two sexual reproductive states, and that recognition of this may promote our study and understanding of plants.

Migration of a Population

On the basis of Hardy – Weinberg law the problem of migration from the genetic point of view is considered. It is proved the linear differential equation of migratory process of a panmictic population. The phase of the solution of this equation is investigated. On the basis of the carried out analysis the dependence of migration velocity of a population on average time of alternation of generations is found. It is shown that migration of primitive people from Africa to Europe needed alternation the several hundred generations. The dependence of migration velocity of a population on the average area developed by a population for year is investigated. Lacks of the carried out analysis owing to absence of the account of natural selection and inbreeding are marked.

Genetic-Mathematical Modelling of Mutational Processes in a Population

Processes of genetic-mathematical modeling of a population development are considered. A basic distinction in the mathematical description of a family tree and a population is shown. In a family tree alternation of generations has discrete character. In a population there is a continuous alternation of generations. The method of the differential equations is applied for the description of a population. It is shown that mutational process in a population can be described with use of a Green’s function. For radiating influence on a population the universal evolutionary law is found.

Sex and the Single Cryptogam

As Chapter 17 makes clear, the asexualist/sexualists controversy continued even as Johann Hedwig and Karl von Nägel demonstrated the existence of sex in cryptogams by discovering the Alternation of Generations (1782, 1784), hybridizers A. F. Wiegman and Carl Friedrich von Gaertner recieved prestigious prizes for their work, and Giovanni Battista Amici and Adolphe-Theodore Brongniart discovered—and confirmed—the pollen tube. Unconvinced, Matthias Jacob Schleiden, co-founder of the cell theory, insisted that ferns grow asexually from spores, and that spores, not seeds, are the primary units of propagation in seed plants also. He argued (1853) that the entire life-cycle of seed plants is based on duplicative cell divisions that produce seeds entirely by vegetative processes. Following the Aristotelian doctrine that the female parent provides the material substance of the embryo, he concluded pollen must be a female structure that reproduces vegetatively—thus making the case for a unisexual, plants-as-female model.