• Refine Query
  • Source
  • Publication year
  • to
  • Language
  • 1112
  • 1081
  • 407
  • 267
  • 175
  • 111
  • 111
  • 111
  • 111
  • 111
  • 109
  • 78
  • 42
  • 35
  • 32
  • Tagged with
  • 4007
  • 958
  • 348
  • 314
  • 229
  • 228
  • 211
  • 199
  • 192
  • 175
  • 174
  • 167
  • 166
  • 165
  • 164
  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
41

The reproductive physiology of triploid Pacific salmonids

Benfey, Tillmann J. January 1988 (has links)
Triploidy was induced in rainbow trout, Salmo gairdneri Richardson, by heat shock (10 min at 26, 28 or 30°C, applied 1 min after fertilization at 10°C) and in pink salmon, Oncorhynchus gorbuscha Walbaum, and coho salmon, 0. kisutch Walb., by hydrostatic pressure shock (1, 2, 3 or 4 min at 69,000 kPa, applied 15 min after fertilization at 10.5°C). Triploid individuals were identified by the flow cytometric measurement of DNA content of erythrocytes stained with propidium iodide. Gonadosomatic index was reduced to a much greater extent in triploid females than males. Triploid ovaries remained very small, and contained virtually no oocytes. Triploid testes became quite large, but few cells developed beyond the spermatocyte stage. Triploid male rainbow trout had significantly lower spermatocrits than diploids, and their spermatozoa were aneuploid. Growth rates were the same for diploid and triploid rainbow trout, but triploid female pink salmon were smaller than maturing diploid females and diploid and triploid males of the same age. Triploid males of both species developed typical secondary sexual characteristics and had normal endocrine profiles for plasma sex steroids and plasma and pituitary gonadotropin, but their cycle was delayed by about one month. Triploid females developed no secondary sexual characteristics and showed no endocrine signs of maturation, even at the level of the pituitary. Vitellogenin synthesis was induced in immature diploid and triploid coho salmon by the weekly injection of 17β-estradiol. Plasma vitellogenin and pituitary gonadotropin levels were significantly elevated over levels of sham-injected fish, whereas plasma gonadotropin levels were slightly depressed. There was no significant difference between diploids and triploids for any of these results, indicating that normal vitellogenesis is not impaired by triploidy per se. It is concluded that triploids of both sexes are genetically sterile, but that only triploid females do not undergo physiological maturation. Triploid testes develop sufficiently for their steroidogenic cells to become active, which is not the case for triploid ovaries. The occasional cells that pass through the normal meiotic block develop to full maturity in triploid males but not in triploid females, probably due to the absence of the appropriate stimulus to initiate and maintain vitellogenesis. Although triploids of both sexes should make valuable tools for basic research on reproductive physiology, only the females will be useful for practical fish culture to avoid the economically detrimental effects of maturation in fish destined for human consumption. / Science, Faculty of / Zoology, Department of / Graduate
42

Photoperiodic stimulation of sexual maturation in prepuberal boars and reproductive efficiency in sows.

Greenberg, Louise Gail. January 1981 (has links)
No description available.
43

Mean fitness of long-term sexual and asexual populations of Chlamydomonas in benign environments

Renaut, Sébastien January 2004 (has links)
No description available.
44

The Reproductive Biology of STERNOTHERUS MINOR MINOR (Reptilia: Testudines: Kinosternidae) from the Southern Part of its Range in Central Florida

Etchberger, Cory R. 01 January 1984 (has links) (PDF)
Variation in chelonian reproductive patterns is well documented. Previous studies of loggerhead musk turtles (Sternotherus minor) reproduction have not attempted to control for variation in latitude, local population differences, and seasonal variation. The present study attempts to control for these variables by collecting turtles from one population for one reproductive season. The reproductive pattern of S. minor at the southern limit of its range (Central Florida) is compared to those of S. minor studied elsewhere. Both male and female musk turtles mature after five to six years and at approximately 60mm plastron length. No sexual dimorphism in overall body size is evident. There is a significant relationship between testis mass and male body size. Spermatogenesis begins in June as testes begin to enlarge. A peak in the testicular cycle is observed in August and September followed by testicular regression from October through January. A germinal quiescent phase is evident from February through April. Vitellogenesis in females begins in mid-September and the first clutch is laid in late October. Ovipositions continue until mid-June when follicular regression begins. A brief but distinct ovarian quiescent period occurs in August. Mean clutch size is 3.0 (range= 1-5). Clutch size and clutch mass were significantly correlated with body size. Egg length is not significantly correlated with clutch size or plastron length. Four clutches per year are common and some females probably produce five. Reproductive potential and individual reproductive effort are both correlated with body size. Testicular activity peaks six months after a peak in the ovarian cycle. Similarities with other studies of Sternotherus minor include: timing of the reproductive cycles, mean female size, and size and age at maturity. Mean clutch size in Central Florida is significantly larger than elsewhere. This difference is explained by the fact that more females produce three and four eggs. While similarities and differences in reproductive characteristics do exist between Central Florida S. minor and more northern populations, it is clear that those similarities and differences must be interpreted with respect to the methods of data collection used. Annual reproductive potential is enhanced in the Central Florida population. This is explained by greater resource availability which is translated into a greater reproductive output.
45

Reproduction and its seasonal variation in the soupfin shark, Galeorhinus galeus

Theron, Daniel Frederick 12 1900 (has links)
Thesis (MSc)--Stellenbosch University, 2001. / ENGLISH ABSTRACT: The soupfin shark, Galeorhinus galeus, is a circum-global shark species of great economical importance. Their reproductive biology is reasonably well understood, but intraspecific differences between global populations necessitate the study o f populations separately. For this study, 70 male and 74 female specimens were collected along the southwestern coast of South Africa between 34°8'S; 18°27'E and 34°24S; 21°25'E. Morphological changes were described and serum steroid hormone concentrations evaluated by enzyme-linked immunosorbent assay (ELISA) over an almost-complete reproductive cycle. Males display an annual spermatogenic cycle that starts with a significant peak in testosterone concentrations coinciding with the start o f spermatogenesis around April-May. Testosterone levels decrease to reach a minimum in late-winter after which spermiogenesis commences. The prevalence of spermiogenic cysts in the testis increase to reach a peak between February and April, after which mating occurs until about July. Females (aplacentally viviparous) are suggested to have a triennial cycle, similar to the Brazilian & Australian populations, but different from the Californian & Mediterranean populations. During the first year after parturition there is not much reproductive development. The following year sees increased vitellogenic activity in ovarian follicles and development of the oviducal glands and uteri. Ovulation then occurs towards the end of that year. Mating is suggested to precede ovulation by two to three months necessitating sperm storage by the females in their oviducal glands. A 12-15 month gestation ensues and ends in parturition the following summer. Testosterone and estradiol levels fluctuate together and are involved in the preparation for ovulation and oviducal gland development. Progesterone levels show two distinct peaks during the year, in both immature and mature females, and could not yet be functionally linked to any reproductive activity or condition. This cycle is not very tightly synchronised among the South African females. / AFRIKAANSE OPSOMMING: Die vaalhaai, Galeorhinus galeus, kom wereld-wyd voor en is van groot ekonomiese belang in baie lande. Die spesie se voorplantings biologie is reeds redelik deeglik ondersoek maar intraspesifieke verskille tussen verskillende wereld populasies noodsaak die bestudering van populasies afsondelik. Daar is 70 mannetjies en 74 wyfies versamel vir hierdie studie langs die Suid Afrikaanse suid-wes kus tussen 34°8'S; 18°27'E and 34°24S; 21°25'E. Morfologiese veranderinge is beskryf en serum hormoon konsentrasies is ge-evalueer deur middel van die ELISA tegniek vir 'n amper-volledige voortplanting siklus. Mannetjies vertoon 'n jaarlikse spermatogeniese siklus met 'n piek in testosteroon konsentrasies aan die begin van spermatogenese in April-Mei. Hiema daal testosteroon vlakke tot minimum vlakke in die laat-winter, waama spermiogenese begin. Die proporsie spermiogeniese siste in die testis vermeerder tot maksimum waardes bereik word tussen Februarie en April. Paring vind hiema plaas tot omtrent Julie. Wyfies (aplasentaal vivipaar) het 'n voorgestelde driejaarlikse siklus, soortgelyk aan die Brasiliaanse & Australiese populasies maar anders as die Kalifomiese & Mediterreense populasies. Tydens die eerste jaar na bevalling vind daar nie veel voortplantings ontwikkeling plaas nie. Die daaropvolgende jaar verhoog vitellogeniese aktiwiteit in die ovarium follikels en ontwikkeling van die dopkliere en uteri. Ovulasie vind nader aan die einde van hierdie jaar plaas. Paring vind heel moontlik twee tot drie maande voor ovulasie plaas, wat noodsaak dat die wyfies sperm moet stoor in hulle dopkliere. 'n 12- 15 maande dratyd begin na ovulasie en eindig in geboorte teen die volgende somer. Testosteroon en estradiol konsentrasies fluktueer saam en speel 'n rol tydens voorbereidings vir ovulasie en dopklier ontwikkeling. Progesteroon vlakke piek twee maal gedurende die jaar, in beide onvolwasse en volwasse wyfies, maar dit kon nog nie funksioneel geassosieer word met enige voortplantings aktiwiteit of toestand nie. Die siklus is nie baie nou gesinkroniseer onder die Suid Afrikaanse wyfies nie.
46

MATERNAL REPRODUCTIVE INVESTMENT IN THE DESERT ANNUAL LESQUERELLA GORDONII: REGULATION AND PATTERNS.

Delph, Lynda Ferrell. January 1983 (has links)
No description available.
47

A study of spontaneous rosette formation in the bovine female during the immediate postestral period

Fahmi, Hisham Ahmad January 2011 (has links)
Typescript (photocopy). / Digitized by Kansas Correctional Industries
48

Estrogen regulation of testicular function in the adult ram

Melnyk, Peter M. (Peter Michael) January 1989 (has links)
No description available.
49

Self-cleavage of plant pathogenic RNAs

Forster, Anthony Carlyle. January 1987 (has links) (PDF)
Includes bibliography.
50

Interactions between thyroid hormones and reproductive function in prepubertal and sexually mature merino rams / by Yallampalli Chandrasekhar

Chandrasekhar, Yallampalli January 1985 (has links)
Includes bibliographical references (leaves 191-207) / xiv, 207 leaves : ill ; 30 cm. / Title page, contents and abstract only. The complete thesis in print form is available from the University Library. / Examines the interactions between thyroid hormones and male reproductive function in mature, prepubertal and post pubertal Merino rams. Hypothyroidism or hyperthyroidism was induced in these rams for 8-10 weeks and their reprodroductive endocrine axis and testis functions were assessed. / Thesis (Ph.D.)--University of Adelaide, Dept. of Animal Sciences, 1986

Page generated in 0.0825 seconds