Willows called sallows and osiers, form the genus Salix, around 400 species of deciduous trees and shrubs, found on moist soils in cold and temperate regions of the Northern Hemisphere. Most species are known as willow, but some narrow-leaved shrub species are called osier, some broader-leaved species are referred to as sallow; some willows are creeping shrubs. Willows all have abundant watery bark sap, charged with salicylic acid, soft pliant, tough wood, slender branches, large, fibrous stoloniferous roots; the roots are remarkable for their toughness and tenacity to live, roots sprout from aerial parts of the plant. The leaves are elongated, but may be round to oval with serrated edges. Most species are deciduous. All the buds are lateral; the buds are covered by a single scale. The bud scale is fused into a cap-like shape, but in some species it wraps around and the edges overlap; the leaves are simple, feather-veined, linear-lanceolate. They are serrate, rounded at base, acute or acuminate; the leaf petioles are short, the stipules very conspicuous, resembling tiny, round leaves, sometimes remaining for half the summer.
On some species, they are small and caducous. In color, the leaves show a great variety of greens. Willows are dioecious, with male and female flowers appearing as catkins on separate plants; the staminate flowers are without either calyx with corolla. This scale is square and hairy; the anthers are orange or purple after the flower opens. The filaments are threadlike pale brown, bald; the pistillate flowers are without calyx or corolla, consist of a single ovary accompanied by a small, flat nectar gland and inserted on the base of a scale, borne on the rachis of a catkin. The ovary is one-celled, the style two-lobed, the ovules numerous. All willows take root readily from cuttings or where broken branches lie on the ground; the few exceptions include the goat peachleaf willow. One famous example of such growth from cuttings involves the poet Alexander Pope, who begged a twig from a parcel tied with twigs sent from Spain to Lady Suffolk; this twig was planted and thrived, legend has it that all of England's weeping willows are descended from this first one.
Willows are planted on the borders of streams so their interlacing roots may protect the bank against the action of the water. The roots are much larger than the stem which grows from them. Willows have a wide natural distribution from the tropics to the arctic zones and are extensively cultivated around the world. Willows are cross-compatible, numerous hybrids occur, both and in cultivation. A well-known ornamental example is the weeping willow, a hybrid of Peking willow from China and white willow from Europe; the hybrid cultivar'Boydii' has gained the Royal Horticultural Society's Award of Garden Merit. Numerous cultivars of Salix L. have been named over the centuries. New selections of cultivars with superior technical and ornamental characteristics have been chosen deliberately and applied to various purposes. Most Salix has become an important source for bioenergy production and for various ecosystem services; the first edition of the Checklist for Cultivars of Salix L. was compiled in 2015, which includes 854 cultivar epithets with accompanying information.
The International Poplar Commission of the FAO UN holds the International Cultivar Registration Authority for the genus Salix. The ICRA for Salix produces and maintains The International Register of Cultivars of Salix L.. Willows are used as food plants by the larvae of some Lepidoptera species, such as the mourning cloak butterfly. Ants, such as wood ants, are common on willows inhabited by aphids, coming to collect aphid honeydew, as sometimes do wasps. A small number of willow species were planted in Australia, notably as erosion-control measures along watercourses, they are now regarded as invasive weeds which occupy extensive areas across southern Australia and are considered'Weeds of National Significance'. Many catchment management authorities are replacing them with native trees. Substantial research undertaken from 2006 has identified that willows inhabit an unoccupied niche when they spread across the bed of shallow creeks and streams and if removed, there is a potential water saving of up to 500 ML/per year per hectare of willow canopy area, depending on willow species and climate zone.
This water could benefit the environment or provision of local water resources during dry periods. To aid management of willows, a remote sensing method has been developed to map willow area along and in streams a
The Siphonophorae or Siphonophora, the siphonophores, are an order of the hydrozoans, a class of marine animals belonging to the phylum Cnidaria. According to the World Register of Marine Species, the order contains 188 species. Although a siphonophore may appear to be a single organism, each specimen is in fact a colonial organism composed of small individual animals called zooids that have their own special function for survival. Most colonies are long, transparent floaters living in the pelagic zone; some siphonophores, such as the venomous Portuguese man o' war and the Indo-Pacific man o' war, superficially resemble jellyfish. Another species of siphonophore, Praya dubia, is one of the longest animals in the world, with a body length of 40–50 m; the term originates from the Greek siphōn'tube' + pherein'to bear'. Siphonophores are of special scientific interest because they are composed of medusoid and polypoid zooids that are morphologically and functionally specialized; each zooid is an individual organism, but its integration with others is so strong that the colony attains the function of a larger organism.
Indeed, most of the zooids are so specialized, they lack the ability to survive on their own. This is somewhat analogous to the function of multicellular organisms. Like other hydrozoans, certain siphonophores can emit light. A siphonophore of the genus Erenna has been discovered at a depth of around 1,600 m off the coast of Monterey, California; the individuals from these colonies are strung together like a feather boa. They prey on small animals using stinging cells. Among the stinging cells are stalks with red glowing ends; the tips twitch forth, creating a twinkling effect. Twinkling red lights are thought to attract the small fish eaten by these siphonophores. While many sea animals produce blue and green bioluminescence, this siphonophore was only the second lifeform found to produce a red light. Due to their specialized colonies, siphonophores have long misled scientists, they were for a long time believed to be a distinct group, but now are known to have evolved from simpler colonial hydrozoans similar to those in the orders Anthoathecata and Leptothecata.
They are now united with these in the subclass Hydroidolina. The Siphonophorae have long fascinated scientists due to their dramatic appearance, as well as the large size and dangerous sting of several species. Compared to their relatives, their systematics are straightforward: Suborder Calycophorae Abylidae Agassiz, 1862 Clausophyidae Totton, 1965 Diphyidae Quoy & Gaimard, 1827 Hippopodiidae Kölliker, 1853 Prayidae Kölliker, 1853 Sphaeronectidae Huxley, 1859 Tottonophyidae Pugh, Dunn & Haddock, 2018 Suborder Cystonectae Physaliidae Brandt, 1835 Rhizophysidae Brandt, 1835 Suborder Physonectae Agalmatidae Brandt, 1834 Apolemiidae Huxley, 1859 Cordagalmatidae Pugh, 2016 Erennidae Pugh, 2001 Forskaliidae Haeckel, 1888 Physophoridae Eschscholtz, 1829 Pyrostephidae Moser, 1925 Resomiidae Pugh, 2006 Rhodaliidae Haeckel, 1888 Stephanomiidae Huxley, 1859 Ernst Haeckel described a number of siphonophores, several plates from his Kunstformen der Natur depict members of the taxon: Mapstone, Gillian M..
Siphonophora of Canadian Pacific waters. Ottawa: NRC Research Press. ISBN 978-0-660-19843-9. PinkTentacle.com: Siphonophore: Deep-sea superorganism. Retrieved 2009-MAY-23. Dunn, Casey. "Siphonophores". Siphonophores. N/a. Retrieved 19 September 2014. Scubamedia.de. "Tauchen in Norwegen - Kvasefjord". YouTube. Scubamedia.de. Retrieved 19 September 2014. Pinktentacle3. "Siphonophore". YouTube. Retrieved 19 September 2014. "Stunning Siphonophore Sighting". Nautilus Live: Explore the ocean LIVE with Dr. Robert Ballard and the Corps of Exploration. Ocean Exploration Trust. 27 June 2014. Retrieved 18 September 2014."Deep sea siphonophore" YouTube. Imaged by the NOAA Okeanos Explorer on March 14, 2017 at 1,560 meters west of Winslow Reef complex. Retrieved 28 January 2018
Animals are multicellular eukaryotic organisms that form the biological kingdom Animalia. With few exceptions, animals consume organic material, breathe oxygen, are able to move, can reproduce sexually, grow from a hollow sphere of cells, the blastula, during embryonic development. Over 1.5 million living animal species have been described—of which around 1 million are insects—but it has been estimated there are over 7 million animal species in total. Animals range in length from 8.5 millionths of a metre to 33.6 metres and have complex interactions with each other and their environments, forming intricate food webs. The category includes humans, but in colloquial use the term animal refers only to non-human animals; the study of non-human animals is known as zoology. Most living animal species are in the Bilateria, a clade whose members have a bilaterally symmetric body plan; the Bilateria include the protostomes—in which many groups of invertebrates are found, such as nematodes and molluscs—and the deuterostomes, containing the echinoderms and chordates.
Life forms interpreted. Many modern animal phyla became established in the fossil record as marine species during the Cambrian explosion which began around 542 million years ago. 6,331 groups of genes common to all living animals have been identified. Aristotle divided animals into those with those without. Carl Linnaeus created the first hierarchical biological classification for animals in 1758 with his Systema Naturae, which Jean-Baptiste Lamarck expanded into 14 phyla by 1809. In 1874, Ernst Haeckel divided the animal kingdom into the multicellular Metazoa and the Protozoa, single-celled organisms no longer considered animals. In modern times, the biological classification of animals relies on advanced techniques, such as molecular phylogenetics, which are effective at demonstrating the evolutionary relationships between animal taxa. Humans make use of many other animal species for food, including meat and eggs. Dogs have been used in hunting, while many aquatic animals are hunted for sport.
Non-human animals have appeared in art from the earliest times and are featured in mythology and religion. The word "animal" comes from the Latin animalis, having soul or living being; the biological definition includes all members of the kingdom Animalia. In colloquial usage, as a consequence of anthropocentrism, the term animal is sometimes used nonscientifically to refer only to non-human animals. Animals have several characteristics. Animals are eukaryotic and multicellular, unlike bacteria, which are prokaryotic, unlike protists, which are eukaryotic but unicellular. Unlike plants and algae, which produce their own nutrients animals are heterotrophic, feeding on organic material and digesting it internally. With few exceptions, animals breathe oxygen and respire aerobically. All animals are motile during at least part of their life cycle, but some animals, such as sponges, corals and barnacles become sessile; the blastula is a stage in embryonic development, unique to most animals, allowing cells to be differentiated into specialised tissues and organs.
All animals are composed of cells, surrounded by a characteristic extracellular matrix composed of collagen and elastic glycoproteins. During development, the animal extracellular matrix forms a flexible framework upon which cells can move about and be reorganised, making the formation of complex structures possible; this may be calcified, forming structures such as shells and spicules. In contrast, the cells of other multicellular organisms are held in place by cell walls, so develop by progressive growth. Animal cells uniquely possess the cell junctions called tight junctions, gap junctions, desmosomes. With few exceptions—in particular, the sponges and placozoans—animal bodies are differentiated into tissues; these include muscles, which enable locomotion, nerve tissues, which transmit signals and coordinate the body. There is an internal digestive chamber with either one opening or two openings. Nearly all animals make use of some form of sexual reproduction, they produce haploid gametes by meiosis.
These fuse to form zygotes, which develop via mitosis into a hollow sphere, called a blastula. In sponges, blastula larvae swim to a new location, attach to the seabed, develop into a new sponge. In most other groups, the blastula undergoes more complicated rearrangement, it first invaginates to form a gastrula with a digestive chamber and two separate germ layers, an external ectoderm and an internal endoderm. In most cases, a third germ layer, the mesoderm develops between them; these germ layers differentiate to form tissues and organs. Repeated instances of mating with a close relative during sexual reproduction leads to inbreeding depression within a population due to the increased prevalence of harmful recessive traits. Animals have evolved numerous mechanisms for avoiding close inbreeding. In some species, such as the splendid fairywren, females benefit by mating with multiple males, thus producing more offspring of higher genetic quality; some animals are capable of asexual reproduction, which results
Ginkgo is a genus of unusual non-flowering plants. The scientific name is used as the English name; the order to which it belongs, first appeared in the Permian, 270 million years ago derived from "seed ferns" of the order Peltaspermales, now only contains this single genus and species. The rate of evolution within the genus has been slow, all its species had become extinct by the end of the Pliocene; the relationships between ginkgos and other groups of plants are not resolved. The ginkgo is a living fossil, with fossils similar to modern ginkgo from the Permian, dating back 270 million years; the most plausible ancestral group for the order Ginkgoales is the Pteridospermatophyta known as the "seed ferns" the order Peltaspermales. The closest living relatives of the clade are the cycads, which share with the extant G. biloba the characteristic of motile sperm. Fossils attributable to the genus Ginkgo first appeared in the Early Jurassic, the genus diversified and spread throughout Laurasia during the middle Jurassic and Early Cretaceous.
It declined in diversity as the Cretaceous progressed with the extinction of species such as Ginkgo huolinhensis, by the Palaeocene, only a few Ginkgo species, Ginkgo cranei and Ginkgo adiantoides, remained in the Northern Hemisphere, while a markedly different form persisted in the Southern Hemisphere. At the end of the Pliocene, Ginkgo fossils disappeared from the fossil record everywhere except in a small area of central China, where the modern species survived, it is doubtful whether the Northern Hemisphere fossil species of Ginkgo can be reliably distinguished. Given the slow pace of evolution and morphological similarity between members of the genus, there may have been only one or two species existing in the Northern Hemisphere through the entirety of the Cenozoic: present-day G. biloba and G. gardneri from the Palaeocene of Scotland. At least morphologically, G. gardneri and the Southern Hemisphere species are the only known post-Jurassic taxa that can be unequivocally recognised. The remainder may have been subspecies.
The implications would be that G. biloba had occurred over an wide range, had remarkable genetic flexibility and, though evolving genetically, never showed much speciation. While it may seem improbable that a species may exist as a contiguous entity for many millions of years, many of the ginkgo's life-history parameters fit; these are: extreme longevity. Modern-day G. biloba grows best in well-watered and drained environments, the similar fossil Ginkgo favoured similar environments. Ginkgo therefore presents an "ecological paradox" because, while it possesses some favourable traits for living in disturbed environments, many of its other life-history traits are the opposite of those exhibited by modern plants that thrive in disturbed settings. Given the slow rate of evolution of the genus, it is possible that Ginkgo represents a pre-angiosperm strategy for survival in disturbed streamside environments. Ginkgo evolved in an era before flowering plants, when ferns and cycadeoids dominated disturbed streamside environments, forming a low, shrubby canopy.
The large seeds of Ginkgo and its habit of "bolting"—growing to a height of 10 metres before elongating its side branches—may be adaptations to such an environment. Diversity in the genus Ginkgo dropped through the Cretaceous at the same time the flowering plants were on the rise, which supports the notion that flowering plants, with their better adaptations to disturbance, displaced Ginkgo and its associates over time. Ginkgo has been used for classifying plants with leaves that have more than four veins per segment, while Baiera for those with less than four veins per segment. Sphenobaiera has been used to classify plants with broadly wedge-shaped leaves that lacks distinct leaf stems. Trichopitys is distinguished by having multiple-forked leaves with cylindrical, thread-like ultimate divisions; as of February 2013, molecular phylogenetic studies have produced at least six different placements of Ginkgo relative to cycads, conifers and angiosperms. The two most common are that Ginkgo is a sister to a clade composed of conifers and gnetophytes or that Ginkgo and cycads form a clade within the gymnosperms.
A 2013 study examined the reasons for the discrepant results, concluded that the best support was for the monophyly of Ginkgo and cycads
A sporophyte is the diploid multicellular stage in the life cycle of a plant or alga. It develops from the zygote produced when a haploid egg cell is fertilized by a haploid sperm and each sporophyte cell therefore has a double set of chromosomes, one set from each parent. All land plants, most multicellular algae, have life cycles in which a multicellular diploid sporophyte phase alternates with a multicellular haploid gametophyte phase. In the seed plants, flowering plants, the sporophyte phase is more prominent than the gametophyte, is the familiar green plant with its roots, stem and cones or flowers. In flowering plants the gametophytes are reduced in size, are represented by the germinated pollen and the embryo sac; the sporophyte produces spores by meiosis, a process known as "reduction division" that reduces the number of chromosomes in each spore mother cell by half. The resulting meiospores develop into a gametophyte. Both the spores and the resulting gametophyte are haploid, meaning they only have one set of chromosomes.
The mature gametophyte produces female gametes by mitosis. The fusion of male and female gametes produces a diploid zygote which develops into a new sporophyte; this cycle is known as alternation of alternation of phases. Bryophytes have a dominant gametophyte phase on which the adult sporophyte is dependent for nutrition; the embryo sporophyte develops by cell division of the zygote within the female sex organ or archegonium, in its early development is therefore nurtured by the gametophyte. Because this embryo-nurturing feature of the life cycle is common to all land plants they are known collectively as the embryophytes. Most algae have dominant gametophyte generations, but in some species the gametophytes and sporophytes are morphologically similar. An independent sporophyte is the dominant form in all clubmosses, ferns and angiosperms that have survived to the present day. Early land plants had sporophytes that produced identical spores but the ancestors of the gymnosperms evolved complex heterosporous life cycles in which the spores producing male and female gametophytes were of different sizes, the female megaspores tending to be larger, fewer in number, than the male microspores.
During the Devonian period several plant groups independently evolved heterospory and subsequently the habit of endospory, in which the gametophytes develop in miniaturized form inside the spore wall. By contrast in exosporous plants, including modern ferns, the gametophytes break the spore wall open on germination and develop outside it; the megagametophytes of endosporic plants such as the seed ferns developed within the sporangia of the parent sporophyte, producing a miniature multicellular female gametophyte complete with female sex organs, or archegonia. The oocytes were fertilized in the archegonia by free-swimming flagellate sperm produced by windborne miniaturized male gametophytes in the form of pre-pollen; the resulting zygote developed into the next sporophyte generation while still retained within the pre-ovule, the single large female meiospore or megaspore contained in the modified sporangium or nucellus of the parent sporophyte. The evolution of heterospory and endospory were among the earliest steps in the evolution of seeds of the kind produced by gymnosperms and angiosperms today.
P. Kenrick & P. R. Crane The origin and early evolution of plants on land. Nature 389, 33-39. T. N. Taylor, H. Kerp and H. Hass Life history biology of early land plants: Deciphering the gametophyte phase. Proceedings of the National Academy of Sciences 102, 5892-5897. P. R. Bell & A. R. Helmsley Green plants, their Origin and Diversity. Cambridge University Press ISBN 0-521-64673-1
The Marchantiophyta are a division of non-vascular land plants referred to as hepatics or liverworts. Like mosses and hornworts, they have a gametophyte-dominant life cycle, in which cells of the plant carry only a single set of genetic information, it is estimated. Some of the more familiar species grow as a flattened leafless thallus, but most species are leafy with a form much like a flattened moss. Leafy species can be distinguished from the similar mosses on the basis of a number of features, including their single-celled rhizoids. Leafy liverworts differ from most mosses in that their leaves never have a costa and may bear marginal cilia. Other differences are not universal for all mosses and liverworts, but the occurrence of leaves arranged in three ranks, the presence of deep lobes or segmented leaves, or a lack of differentiated stem and leaves all point to the plant being a liverwort. Liverworts are small from 2–20 mm wide with individual plants less than 10 cm long, are therefore overlooked.
However, certain species may cover large patches of ground, trees or any other reasonably firm substrate on which they occur. They are distributed globally in every available habitat, most in humid locations although there are desert and Arctic species as well; some species can be a weed in gardens. Most liverworts are small, measuring from 2–20 millimetres wide with individual plants less than 10 centimetres long, so they are overlooked; the most familiar liverworts consist of a prostrate, ribbon-like or branching structure called a thallus. However, most liverworts produce flattened stems with overlapping scales or leaves in two or more ranks, the middle rank is conspicuously different from the outer ranks. Liverworts can most reliably be distinguished from the similar mosses by their single-celled rhizoids. Other differences are not universal for all mosses and all liverworts. Unlike any other embryophytes, most liverworts contain unique membrane-bound oil bodies containing isoprenoids in at least some of their cells, lipid droplets in the cytoplasm of all other plants being unenclosed.
The overall physical similarity of some mosses and leafy liverworts means that confirmation of the identification of some groups can be performed with certainty only with the aid of microscopy or an experienced bryologist. Liverworts have a gametophyte-dominant life cycle, with the sporophyte dependent on the gametophyte. Cells in a typical liverwort plant each contain only a single set of genetic information, so the plant's cells are haploid for the majority of its life cycle; this contrasts with the pattern exhibited by nearly all animals and by most other plants. In the more familiar seed plants, the haploid generation is represented only by the tiny pollen and the ovule, while the diploid generation is the familiar tree or other plant. Another unusual feature of the liverwort life cycle is that sporophytes are short-lived, withering away not long after releasing spores. In other bryophytes, the sporophyte is persistent and disperses spores over an extended period; the life of a liverwort starts from the germination of a haploid spore to produce a protonema, either a mass of thread-like filaments or else a flattened thallus.
The protonema is a transitory stage in the life of a liverwort, from which will grow the mature gametophore plant that produces the sex organs. The male organs produce the sperm cells. Clusters of antheridia are enclosed by a protective layer of cells called the perigonium; as in other land plants, the female organs are known as archegonia and are protected by the thin surrounding perichaetum. Each archegonium has a slender hollow tube, the "neck", down which the sperm swim to reach the egg cell. Liverwort species may be either monoicous. In dioicous liverworts and male sex organs are borne on different and separate gametophyte plants. In monoicous liverworts, the two kinds of reproductive structures are borne on different branches of the same plant. In either case, the sperm must move from the antheridia where they are produced to the archegonium where the eggs are held; the sperm of liverworts is biflagellate, i.e. they have two tail-like flagellae that enable them to swim short distances, provided that at least a thin film of water is present.
Their journey may be assisted by the splashing of raindrops. In 2008, Japanese researchers discovered that some liverworts are able to fire sperm-containing water up to 15 cm in the air, enabling them to fertilize female plants growing more than a metre from the nearest male; when sperm reach the archegonia, fertilisation occurs, leading to the production of a diploid sporophyte. After fertilisation, the immature sporophyte within the archegonium develops three distinct regions: a foot, which both anchors the sporophyte in place and receives nutrients from its "mother" plant, a spherical or ellipsoidal capsule, inside which the spores will be produced for dispersing to new locations, a seta which lies between the other two
Karyogamy is the final step in the process of fusing together two haploid eukaryotic cells, refers to the fusion of the two nuclei. Before karyogamy, each haploid cell has one complete copy of the organism's genome. In order for karyogamy to occur, the cell membrane and cytoplasm of each cell must fuse with the other in a process known as plasmogamy. Once within the joined cell membrane, the nuclei are referred to as pronuclei. Once the cell membranes and pronuclei fuse together, the resulting single cell is diploid, containing two copies of the genome; this diploid cell, called a zygote or zygospore can enter meiosis, or continue to divide by mitosis. Mammalian fertilization uses a comparable process to combine haploid sperm and egg cells to create a diploid fertilized egg; the term karyogamy comes from the Greek karyo- meaning "nut" and γάμος gamos, meaning "marriage". Haploid organisms such as fungi and algae can have complex cell cycles, in which the choice between sexual or asexual reproduction is fluid, influenced by the environment.
Some organisms, in addition to their usual haploid state, can exist as diploid for a short time, allowing genetic recombination to occur. Karyogamy can occur within either mode of reproduction: in somatic cells. Thus, karyogamy is the key step in bringing together two sets of different genetic material which can recombine during meiosis. In haploid organisms that lack sexual cycles, karyogamy can be an important source of genetic variation during the process of forming somatic diploid cells. Formation of somatic diploids circumvents the process of gamete formation during the sexual reproduction cycle and instead creates variation within the somatic cells of an developed organism, such as a fungus; the role of karyogamy in sexual reproduction can be demonstrated most by single-celled haploid organisms such as the algae of genus Chlamydomonas or the yeast Saccharomyces cerevisiae. Such organisms exist in a haploid state, containing only one set of chromosomes per cell. However, the mechanism remains the same among all haploid eukaryotes.
When subjected to environmental stress, such as nitrogen starvation in the case of Chlamydomonas, cells are induced to form gametes. Gamete formation in single-celled haploid organisms such as yeast is called sporulation, resulting in many cellular changes that increase resistance to stress. Gamete formation in multicellular fungi occurs in the gametangia, an organ specialized for such a process by meiosis; when opposite mating types meet, they are induced to leave the vegetative cycle and enter the mating cycle. In yeast, there are two mating types, a and α. In fungi, there can be two, four, or up to 10,000 mating types, depending on the species. Mate recognition in the simplest eukaryotes is achieved through pheromone signaling, which induces shmoo formation and begins the process of microtubule organization and migration. Pheromones used in mating type recognition are peptides, but sometimes trisporic acid or other molecules, recognized by cellular receptors on the opposite cell. Notably, pheromone signaling is absent in higher fungi such as mushrooms.
The cell membranes and cytoplasm of these haploid cells fuse together in a process known as plasmogamy. This results in a single cell with two nuclei, known as pronuclei; the pronuclei fuse together in a well regulated process known as karyogamy. This creates a diploid cell known as a zygote, or a zygospore, which can enter meiosis, a process of chromosome duplication and cell division, to create four new haploid gamete cells. One possible advantage of sexual reproduction is that it results in more genetic variability, providing the opportunity for adaptation through natural selection. Another advantage is efficient recombinational repair of DNA damages during meiosis. Thus, karyogamy is the key step in bringing together a variety of genetic material in order to ensure recombination in meiosis; the Amoebozoa is a large group of single-celled species that have been determined to have the machinery for karyogamy and meiosis. Since the Amoeboza branched off early from the eukaryotic family tree, this finding suggests that karyogamy and meiosis were present early in eukaryotic evolution.
The ultimate goal of karyogamy is fusion of the two haploid nuclei. The first step in this process is the movement of the two pronuclei toward each other, which occurs directly after plasmogamy; each pronucleus has a spindle pole body, embedded in the nuclear envelope and serves as an attachment point for microtubules. Microtubules, an important fiber-like component of the cytoskeleton, emerge at the spindle pole body; the attachment point to the spindle pole body marks the minus end, the plus end extends into the cytoplasm. The plus end has normal roles in mitotic division, but during nuclear congression, the plus ends are redirected; the microtubule plus ends attach to the opposite pronucleus, resulting in the pulling of the two pronuclei toward each other. Microtubule movement is mediated by a family of motor proteins known as kinesins, such as Kar3 in yeast. Accessory proteins, such as Spc72 in yeast, act as a glue, connecting the motor protein, spindle pole body and microtubule in a structure known as the half-bridge.
Other proteins, such as Kar9 and Bim1 in yeast, attach to the plus end of the microtubules. They are activated by pheromone signals to attach to the shmoo tip. A shmoo is a projection of the cellular membrane, the site of initial cell fusion in plasmogamy. After plasmogamy, the microtubule plus