DAVID M. PHILLIPS from the Department of Anatomy and the Laboratories for Uman Reproduction and Reproductive Biology, Harvard Medical School, Boston, Massachusetts
Total Page：16
File Type：pdf, Size：1020Kb
REV I E W INSECT SPERM : THEIR STRUCTURE AND MORPHOGENESIS DAVID M. PHILLIPS From the Department of Anatomy and the Laboratories for uman Reproduction and Reproductive Biology, Harvard Medical School, Boston, Massachusetts . Dr. Phillips' present address is the Department of Biology, Washington University ; St. Louis, Missouri 63130 The thorough descriptive studies by the early QRGANIZATION OF THE INSECT TEsr s cytologists Meves (1901, 1907), Retzius (1904, Whine butterflies and moths possess but a single 1909), Bowen (1920, 1922 a-c, 1924), Pollister testis, most species of insects have paired testes (1930), and Johnson (1931) provided later workers covered by a simple epithelium that is frequently with a remarkably accurate picture of the micro- pigmented. Indeed, the bright red, green, yellow, scopic anatomy of insect spermatozoa and of or brown color imparted by this pigmented epi- spermiogenesis in several insect species . Their light thelium is quite helpful in identifying the organ microscope observations were necessarily lacking during dissection . The testicular epithelium in in detail, however, and since the advent of the most insects encloses a number of follicles, each electron microscope, an increasing number of cell delimited by its own epithelium and connected biologists have undertaken to extend our under- independently to the genital duct . The number standing of the structure of insect germ cells. and shape of the follicles varies widely from one Some have confined their attention to spermio- insect group to another (Imms, 1948 ; Wiggles- genesis or the morphology of mature spermatozoa worth, 1953 ; Ross, 1961) . For example, in many of a particular species, while others have been of the Diptera, each testis consists of only one concerned with the comparative morphology of follicle, while in leafhoppers there are many spher- single sperm organelles. Although there are by now many relevant papers of limited scope widely dispersed in the biological literature, there seems Testes were dissected in cold fixative, usually within to be no single reference to which one can turn for a few hours of being collected . Testes were usually an overview of the structure and development of fixed 2-6 hr in 3-60/0 glutaraldehyde (Eastman Kodak Co ., Rochester, N.Y.) and buffered with the insect spermatozoon. In this review an effort is 0.2 M collidine (Eastman Kodak Co.) at pH 7.2-7 .6. made to gain some perspective from a synthesis of Testes were then transferred through several changes these fragmentary accounts supplemented by the of cold buffer (5-30 min) and were postfixed for 1-4 author's own observations based on electron micro- hr in 1 % collidine-buffered OsO 4. After dehydration scopic examination of nearly two hundred insect in alcohol, tissues were embedded in Epon (Luft, species.' 1961), sectioned on a Sorvall MT-1 ultramicrotome (Ivan Sorvall Inc., Norwalk Conn.), were stained in ' The testes of 195 insect species representing 15 3% aqueous uranyl acetate (12-16 hr) and lead orders have been examined . Most of the insects were citrate (Venable and Coggeshall, 1965), and were collected in the vicinity of Boston, Massachusetts . examined in a Siemens Elmiskop I . 243 FIGURE 1 Low magnification electron micrograph of the imaginal testis of the caddis fly Plalycentropus (Limnephilidae) . All spermatozoa in this field appear in near transverse section . This indicates that neighboring cysts are oriented in the same direction . The spermatids within a cyst are also aligned in register so that a transverse section passes through all the cells within the cyst at nearly the same level . For instance, in some cysts all the spermatozoa are transected through the nucleus whereas in other cysts, which are transected farther posteriorly, no nuclei are observed . Most cysts contain exactly 128 (27 ) spermatozoa, which presumably arose from five synchronous mitotic and two synchronous meiotic divi- sions . The finding of nearly exactly 2 7 spermatozoa per cyst suggests that in insects very few germ cells die during spermatogenesis or spermiogenesis . X 7, .500. 2 4 4 THE JOURNAL OF CELL BIOLOGY . VOLUME 44, 1970 ical follicles in a cluster resembling a small bunch are arranged in order of increasing maturity from of grapes . The shape and arrangement of the the periphery to the center of the testis . In grass- multiple elongated follicles in the testis of some hoppers, where the follicles are long and slender, grasshoppers is reminiscent of a bunch of bananas . cysts at the anterior end of the testis contain rela- The follicles contain many cysts, each of which tively early stages of the germ cells, and progres- consists of a clone of germinal cells embedded in sively more mature stages are found at successive large polyvalent epithelial cells (Baccetti and levels toward the posterior end (Lima-de-Faria, Bairati, 1964 ; Cantacuzene, 1968) . The gonial and 1959) . meiotic divisions are synchronous within a given In most species of insects, gonial and meiotic cyst, and the cytoplasmic bridges that result from divisions occur in pupal or nymphal stages of the incomplete cytokinesis in these divisions make each life cycle . Therefore, the imaginal testis contains clone a functional syncytium (Baccetti and Bairati, only spermatids and spermatozoa . In insects that 1964 ; Hoage and Kessel, 1968) . The number of live a very short time as adults, such as mayflies spermatids per cyst is characteristic for each species (Needham et al ., 1935), caddis flies (Ross, 1944), and can be expressed as 2n with "n" usually equal and dark winged fungus gnats (Crouse, 1943 ; to 5, 6, 7, or 8 . The number of divisions occurring 1965), the testis of adults contains only sperma- in a cyst, therefore, appears to be constant for a tozoa. On the other hand, in some species that are given species . Two to the fifth power (i.e ., 32) cells relatively long lived as adults (e .g ., many beetles per cyst are characteristic of coccids (Hughes- and dragonflies), meiotic and even gonial di- Schrader, 1935, 1946; Nur, 1962) and some flies visions occur in the imaginal testis . (Hess and Meyer, 1968) while 2 6 (i .e., 64) cells characterize the cysts of other flies (Hess and THE MITOCHONDRIA OF Meyer, 1968) . The testicular cysts of many caddis INSECT SPERM fly species contain 2 1 (i.e ., 128) spermatids per cyst, or very nearly that number (Phillips, unpub- Light Microscopic Analysis lished) (Fig . 1) . 256 (2 8) spermatids per cyst ap- Early students of insect spermiogenesis were par- pear to be common to most beetles, butterflies, ticularly interested in the complicated, precisely and moths (Phillips, unpublished) . ordered steps by which the numerous small sper- Within a cyst, the interconnected spermatids matocyte mitochondria are transformed into giant differentiate synchronously (Smith, 1916 ; Bairati, mitochondrial derivatives, which occupy most of 1967) . Cysts containing spermatogonia, spermato- the volume of mature insect spermatozoa. Similar cytes, or young spermatids are generally approxi- sequences of mitochondrial reorganizations were mately spherical or polyhedral. As the germ cells described by several investigators working with a elongate, the cysts also elongate and the spermatids number of different species . The most complete become aligned parallel and in register along this studies were those of Bowen (1920, 1922 a, b, 1924), length . The alignment of neighboring spermatids Pollister (1930), and Johnson (1931) . These and in the late stages is, in most cases, nearly perfect, so other investigators reported that by telophase of that a transverse section of a cyst containing late the second meiotic division, the previously spher- spermatids transects all the spermatids at ap- ical mitochondria become elongate structures lo- proximately the same level (Fig . I) . The anterior cated in the zwischenko'rper between the two future portions of the heads of late spermatids are usually spermatids . In some species the mitochondria ap- embedded in the polyvalent cells of the cyst wall . pear to be pinched in half by the cleavage furrow Posteriorly the cells are free in the cyst lumen (Meves, 1907 ; Duesberg, 1911 ; Bowen, 1920 ; where they are sometimes surrounded by amor- Johnson, 1931) resulting in an approximately phous or finely granular extracellular material equal distribution of mitochondria to daughter which, in some species, displays considerable elec- cells at the second meiotic division (for discussion tron opacity . see Wilson, 1928) . Cysts within a follicle are generally systemat- Just after meiosis the mitochondria once again ically arranged with respect to the stage of develop- become spherical and are clustered together in one ment of the included germ cells . In insects in which area of the cell . They then begin a complex series the testicular follicles are spherical (leafhoppers, of rearrangements and fusions that lead to their and many beetles, butterflies, and moths), cysts integration into a large spherical mass, which DAVID M . PHILLIPS Insect Sperm : Their Structure and Morphogenesis 24$ Retzius (1904) termed the nebenkern. In some insect The mitochondria remain closely associated with species, the nebenkern is initially shaped like a the flagellum and subsequently elongate parallel ring rather than a sphere, and the mitochondria of to its long axis . In some hemipterans the two mito- this ring nebenkern subsequently take on a flattened chondria spiral about the flagellum during this shape and become arranged into a stack