MALE GAMETES OF ATRACTOMORPHA ECHINATA HOFFMAN (CHLOROPHYCEAE)1

Many naked gametes are produced in each fusiform, male gametangium of Atractomorpha echinata Hoffman and are liberated through irregularly shaped pores in the gametangial wall. They are typically biflagellate, pyriform or fusiform in shape, 6-11 μm long, and only a few micrometers wide. A mature male gamete is characterized by: (i) a nucleus with condensed chromatin and no nucleoli, (ii) a reduced, starch filled chloroplast occupying a posterior position, and (iii) a cup shaped eyespot consisting of a single layer of plastoglobuli. The flagellar apparatus includes two types of flagellar roots alternating in a cruciate pattern. One type consists of two microtubules, while the other consists of microtubules of varying number, usually eight or nine, but rarely as many as eleven. The paired basal bodies are connected anteriorly by a broad, striated distal fiber; there is no dense apical cap as reported in Sphaeroplea sperm. A unique structure, consisting of three layers of small subunits (6–8 nm diameter) arranged in a paracrystalline array, is positioned beneath each basal body. Based on the structure of its male gametes, Atractomorpha clearly demonstrates affinity with the chlorophycean rather than the ulvaphycean line of evolution. Moreover, if phylogenetic affinities for the Sphaeropleaceae are to be sought among other groups of green algae, the Chlorococcales appears the most promising candidate.     

Gamete attachment process revealed in flowering plant fertilization

Sex-possessing organisms perform sexual reproduction, in which gametes from different sexes fuse to produce offspring. In most eukaryotes, one or both sex gametes are motile, and gametes actively approach each other to fuse. However, in flowering plants, the gametes of both sexes lack motility. Two sperm cells (male gametes) that are contained in a pollen grain are recessively delivered via pollen tube elongation. After the pollen tube bursts, sperm cells are released toward the egg and central cells (female gametes) within an ovule (Fig. 1). The precise mechanism of sperm cell movement after the pollen tube bursts remains unknown. Ultimately, one sperm cell fuses with the egg cell and the other one fuses with the central cell, producing an embryo and an endosperm, respectively. Fertilization in which 2 sets of gamete fusion events occur, called double fertilization, has been known for over 100 y. The fact that each morphologically identical sperm cell precisely recognizes its fusion partner strongly suggests that an accurate gamete interaction system(s) exists in flowering plants.Open in a separate windowFigure 1.Illustration of the fertilization process in flowering plants. First, each pollen tube accesses an ovule containing egg and central cells. Next, the 2 sperm cells face the female gametes in the ovule after the pollen tube bursts. Finally, each sperm cell simultaneously fuses with either egg or central cell.     

Ultrastructural study of spermatogenesis in Oscarella lobularis (Porifera,Demospongiae)

Summary

The various phases of spermatogenesis in the demosponge Oscarella lobularis were studied by electron microscopy. Spermatogenesis occurs within spermatic cysts, which are presumed to derive from choanocyte chambers by transformation of choanocytes into spermatogonia. Germ cells develop asynchronously within spermatocysts, and cytoplasmic bridges, indicating incomplete cells division, connect several germ cells. Attached spermatogonia suggest gonial generations. Spermatocytes I typically show the presence of synaptonemal complexes indicating meiotic divisions. Spermatocytes II have a small size probably because of the meiotic divisions of spermatocytes I. Spermatids are characterized by an acrosome, a big mitochondrion and a peripheral sheath of condensed chromatin surrounding a clearer central area in the nucleus. The mature spermatozoon shows a lateral flagellum and a flattened acrosome capping the nucleus. The phylogenetic implications of some features of the spermatozoon are suggested.     

Spermatogenesis and sperm—spermatheca relations in Spirorbis spirorbis (L.)

The differentiation from early spermatid to spermatozoon is described with special emphasis on the formation of the helix of chromatin and mitochondrial junctions. The role of microtubules in morphogenesis is discussed.

New observations on the role of the recently described spermatheca are presented; phagocytosis and digestion of spermatozoa are proven, and the various origins of the sperm found in the spermatheca are specified.     

DNA甲基化在精子发生中的作用

精子发生(spermatogenesis)是一个高度特化的细胞复杂分化过程,其中DNA二核苷酸CpG甲基化变化与基因转录激活、染色质改构以及遗传印记相关,并且该甲基化与基因表达之间的关系是非直接的,其可通过染色质结构的改变或DNA与蛋白质的相互作用来介导。本文着重介绍精子发生过程中DNA甲基化及其跨代遗传风险、DNA甲基转移酶的调控机制以及DNA甲基化与男性不育之间的关系等,为不育症的防治、精子表观遗传质量评价以及降低辅助生殖技术后代表观遗传疾病风险等提供基础资料。     

拟目乌贼精子发生的超微结构

应用扫描电镜和透射电镜观察了拟目乌贼(Sepia lycidas)精子的发生过程和超微结构。结果表明,精子发生经历了精原细胞、初级精母细胞、次级精母细胞、精细胞和成熟精子五个阶段,其中精细胞可以分为Ⅰ、Ⅱ、Ⅲ、Ⅳ、Ⅴ五个时期,精细胞Ⅱ期又可分为前期和后期。细胞核经历了一个横向收缩、纵向拉长的过程,由圆形或椭圆形,变为不规则的纺锤形、稍弯曲的长柱形;核内染色质由絮状,变为絮块状、致密颗粒状、细纤维状、粗纤维状和片层状,直至高电子密度均质状;顶体由圆形,变为头盔形、圆锥形、倒“U”字形,直至子弹头形;线粒体由空泡状经过融合和迁移,变为内嵴丰富的鸡冠状,形成不完全包围鞭毛的线粒体距。成熟精子全长101.28μm,由头部和尾部组成,头部为稍弯曲的长柱形,长7.73μm,宽1.51μm,由顶体和细胞核组成;尾部细长,为93.18μm,为典型的“9 2”结构,由中段、主段和末段三部分组成。     

The ultrastructure of the ascus apical apparatus of some Dermateaceae (Helotiales)

The ascus apical apparatus ultrastructure of 12 species of the Dermateaceae (Helotiales, Ascomycota) was studied. Its development and details are described on the basis of transmission electron microscopy micrographs for all studied species. A majority of them, 8 species belonging to the genera Belonopsis, Mollisia, and Pyrenopeziza, show considerable similarity in the structure of their ascus apical apparatus. Two species, Mollisia melaleuca and Mollisia ramealis, show some deviations from this type, suggesting that the genus Mollisia is not fully homogeneous. Two other species, Podophacidium xanthomelum and Pezicula cinnamomea represent two ascus apical apparatus structure types of their own, which are remarkably different from the Mollisia type. The possible taxonomic implications arising from ascus apical apparatus diversity in the Dermateaceae are discussed.     

Sporulation of Marteilioides branchialis N. Sp. (Paramyxea) in the Sydney Rock Oyster, Saccostrea commercialis: An Electron Microscope Study

ABSTRACT The ultrastructure of sporulation of a new parasite, Marteilioides branchialis (Paramyxea), in the Sydney Rock oyster, Saccostrea commercialis , is described. The development is typical of other Paramyxea whereby a stem cell internally cleaves a secondary cell contained within a vacuole. It differs from other species in the phylum in that each secondary cell produces a single spore composed of two concentric cells, one within a vacuole of the other. This type of sporulation represents the simplest of all known Paramyxea. Infection results in focal gill lesions and was observed concurrently with an epizootic of another paramyxean, Marteilia sydneyi .     

Spermatogenesis and the structure of the testes in Isodiametra pulchra (Isodiametridae,Acoela)

Boone M., Willems M., Claeys M. and Artois, T. 2011. Spermatogenesis and the structure of the testes in Isodiametra pulchra (Isodiametridae, Acoela). —Acta Zoologica (Stockholm) 92 : 101–108. Spermatogenesis and the structure of the testes were studied ultrastructurally in Isodiametra pulchra (Smith and Bush, Transactions of the American Microscopical Society 1991; 110: 12; Hooge and Tyler, Journal of zoological systematics and evolutionary research 2005; 43: 100). The testes are paired, compact, non‐follicular and lie dorsally and dorso‐laterally to the paired ovaries, partially enfolding them. All stages of spermatogenesis, including spermiogenesis, are described at the ultrastructural level and their spatial organization within the testes is discussed. The cells at the early stages of spermatogenesis (spermatogonia and spermatocytes) are located on the dorsal and dorso‐lateral sides of the testes, while the late stages (spermatids and filiform spermatozoa with 9+2 axonemes) lie at the ventral and inner periphery of the testes, adjacent to ovaries. All the cell types can be found both at the anterior and the posterior end of the testes. The value of the structure of the testes as a phylogenetic marker is addressed.     

Origins of new male germ-line functions from X-derived autosomal retrogenes in the mouse

Recent literature demonstrates that retrogenes tend to leave the X chromosome and integrate onto the autosomes and evolve male-biased expression patterns. Several selection-based evolutionary mechanisms have been proposed to explain this observation. Testing these selection-based models requires examining the evolutionary history and functional properties of new retrogenes, particularly those that show evidence of directional movement between the X and the autosomes (X-related retrogenes). This includes autosomal retrogenes with parental paralogs on the X chromosome (X-derived autosomal retrogenes) and those retrogenes integrated onto the X chromosomes (X-linked retrogenes). In order to understand why retrogenes tend to move nonrandomly in genomes, we examined the expression patterns and evolutionary mechanisms concerning gene pairs having young retrogenes--originating less than 20 MYA (after mouse-rat split). We demonstrate that these X-derived autosomal retrogenes evolved a more restricted male-biased expression pattern: they are expressed exclusively or predominantly in the testis, in particular, during the late stages of spermatogenesis. In contrast, the parental counterparts have relatively broad expression patterns in various tissues and spermatogenetic stages. We further observed that positive selection is targeting these X-derived autosomal retrogenes with novel male-biased expression patterns. This suggests that such retrogenes evolved new male germ-line functions that may be complementary to the functions of the parental paralogs, which themselves contribute little during spermatogenesis. Such evolutionary changes may be beneficial to the populations. Furthermore, most identified X-related retrogenes have recruited novel adjacent sequences as their untranslated regions (UTRs), suggesting that these UTRs, acquired de novo, may play an important role in establishing new regulatory mechanisms to carry out the new male germ-line functions.     

苔草属果实表皮的超微结构及其在分类学上的意义

报道了国产苔草属,两亚属,７组中的１６种植物在电子显微镜下果实表皮的特征和研究结果。根据种内个体间、组内种间以及亚属间的比较,作者认为在苔草属中,果实表皮显微特征在种内个体间比较稳定,变异很上;同组内种间表现出的较大相似性和不同组间存在的一定差异性。说明在组和组下等级的分类学处理以及在系统位置和亲缘关系的探讨上,果实显微特征具有重要的分类学价值。同时由于是微特征在各间甚至属间相互交叉和渗透,在划分     

Ultrastructural Study of the Development of Pleistophora schubergi Zwölfer, 1927 (Protozoa,Microsporida) in Larvae of the Spruce Budworm,Choristoneura fumiferana and Its Subsequent Taxonomic Change to the Genus Endoreticulatus

This study demonstrates that Pleistophora schubergi Zwölfer, 1927, a microsporidium originally isolated from the midgut epithelium of Nygmia phaeorrhoea Don (Euproctis chrysorrhoea L.) and Porthetria dispar L., and subsequently reported in several other insects including the spruce budworm, Choristoneura fumiferana (the host used in this investigation), does not belong in the genus Pleistophora Gurley, 1893. Pleistophora schubergi lacks the major features that are characteristic of Pleistophora typicalis, the type species of this genus. A comparison of ultrastructural observations reported for the type species of the genus Pleistophora, P. typicalis, and our observations of P. schubergi revealed significant differences. A thick (0.5 μm) amorphous coat, derived from parasite secretions and deposited external to the parasite plasmalemma, surrounds all developmental stages in P. typicalis. Double membranes, derived from host rough endoplasmic reticulum cisternae encircle the parasite plasmalemma of all developmental stages in P. schubergi. The sporophorous vesicle encases the spores in P. typicalis, and originates from the parasite-secreted coat that is present around meronts. In P. schubergi, the host endoplasmic reticulum cisternae form the envelope that surrounds the meronts. Moreover, the sporophorous vesicle envelope in P. typicalis persists around groups of spores, while in P. schubergi this envelope breaks easily to release the spores in the host cytoplasm. By comparing the characteristics of the microsporidium found in the spruce budworm with those of the recently created polysporous genera that sporulate within a vesicle, we found that P. schubergi does belong in the new genus Endoreticulatus Brooks et al. 1988, and consequently rename it Endoreticulatus schubergi (Zwölfer, 1927) n. comb.     

Gametogenesis and the Chromosomes of Meloidogyne nataliei: Not Typical of Other Root-knot Nematodes

Studies of oogenesis and spermatogenesis revealed that Meloidogyne nataliei is a diploid, amphimictic species with four (n), relatively large chromosomes, and possibly with an XX ♀-XY ♂ mechanism of sex determination. It differs considerably from all other amphimictic, or meiotically parthenogenetic, species of Meloidogyne which have 13-18 smaller chromosomes and from Meloidogyne (Hypsoperine) spartinae which has seven. Consequently, the taxonomic position of M. nataliei needs to be re-evaluated. The chromosomes of M. nataliei and their behavior during gametogenesis resemble more closely chromosomes of the genus Heterodera than those of the genus Meloidogyne. This resemblance, however, may not imply a closer phyletic relationship of M. nataliei to heteroderid nematodes.