Gamete

This is an example of anisogamy or heterogamy, the condition in which females and males produce gametes of different sizes (this is the case in humans; the human ovum has approximately 100,000 times the volume of a single human sperm cell). In contrast, isogamy is the state of gametes from both sexes being the same size and shape, and given arbitrary designators for mating type. The name gamete was introduced by the German cytologist Eduard Strasburger. Male and female gametes set the basis for the sexual roles and sexual selection.[4]

gamete

Oogenesis is the process of female gamete formation in animals. This process involves meiosis (including meiotic recombination) occurring in the diploid primary oocyte to produce the haploid ovum. Spermatogenesis is the process of male gamete formation in animals. This process also involves meiosis occurring in the diploid primary spermatocyte to produce the haploid spermatozoon.

It is generally accepted that isogamy is the ancestral state from which anisogamy evolved, although its evolution has left no fossil records.[5] Oogamy also evolved from isogamy through anisogamy.[6][7] There are almost invariably only two gamete types, all analyses showing that intermediate gamete sizes are eliminated due to selection.[8][9] Intermediate sized gametes do not have the same advantages as small or large ones;[10] they do worse than small ones in mobility and numbers, and worse than large ones in supply.[11]

In contrast to a gamete, a diploid somatic cell of an individual contains one copy of the chromosome set from the sperm and one copy of the chromosome set from the egg cell. Consequently, the cells of the offspring have genes potentially capable of expressing characteristics of both the father and the mother, subject to whether they are dominant or recessive. A gamete's chromosomes are not exact duplicates of either of the sets of chromosomes carried in the diploid chromosomes but a mixture of the two.[12]

Artificial gametes, also known as In vitro derived gametes (IVD), stem cell-derived gametes (SCDGs), and In vitro generated gametes (IVG), are gametes derived from stem cells. The use of such artificial gametes would [QUOTE:] "necessarily require IVF techniques".[14] Research shows that artificial gametes may be a reproductive technique for same-sex male couples, although a surrogate mother would still be required for the gestation period.[14] Women who have passed menopause may be able to produce eggs and bear genetically related children with artificial gametes.[14] Robert Sparrow wrote, in the Journal of Medical Ethics, that embryos derived from artificial gametes could be used to derive new gametes and this process could be repeated to create multiple human generations in the laboratory.[15] This technique could be used to create cell lines for medical applications and for studying the heredity of genetic disorders.[15] Additionally, this technique could be used for human enhancement by selectively breeding for a desired genome or by using recombinant DNA technology to create enhancements that have not arisen in nature.[15]

Plants which reproduce sexually also produce gametes. However, since plants have a life cycle involving alternation of diploid and haploid generations some differences exist. Plants use meiosis to produce spores that develop into multicellular haploid gametophytes which produce gametes by mitosis. The sperm are formed in an organ known as the antheridium and the egg cells in a flask-shaped organ called the archegonium. In flowering plants, the female gametophyte is produced inside the ovule within the ovary of the flower. When mature, the haploid gametophyte produces female gametes which are ready for fertilization. The male gametophyte is produced inside a pollen grain within the anther. When a pollen grain lands on a mature stigma of a flower it germinates to form a pollen tube that grows down the style into the ovary of the flower and then into the ovule. The pollen then produces sperm by mitosis and releases them for fertilization.[clarification needed]

The relative energetic investment in reproduction between the sexes forms the basis of sexual selection and life history theories in evolutionary biology. It is often assumed that males invest considerably less in gametes than females, but quantifying the energetic cost of gamete production in both sexes has remained a difficult challenge. For a broad diversity of species (invertebrates, reptiles, amphibians, fishes, birds, and mammals), we compared the cost of gamete production between the sexes in terms of the investment in gonad tissue and the rate of gamete biomass production. Investment in gonad biomass was nearly proportional to body mass in both sexes, but gamete biomass production rate was approximately two to four orders of magnitude higher in females. In both males and females, gamete biomass production rate increased with organism mass as a power law, much like individual metabolic rate. This suggests that whole-organism energetics may act as a primary constraint on gamete production among species. Residual variation in sperm production rate was positively correlated with relative testes size. Together, these results suggest that understanding the heterogeneity in rates of gamete production among species requires joint consideration of the effects of gonad mass and metabolism.

Efforts to understand the considerable heterogeneity in gamete production within and between the sexes has generally taken place in the context of life history theory. Among males, differences among species in the size of gonads and/or sperm, and rates of sperm production, are typically attributed to the intensity of postcopulatory sexual selection in the form of sperm competition: Males experiencing greater sperm competition are expected to invest relatively more biomass in gonads and produce sperm at a relatively higher rate [3], [7], [8], [9], [10]. In females, no similar theory has been proposed to explain differences in the energy expended for gonad biomass and gamete production, but the expectation from life history theory is that females produce the optimal size and number of gametes at a rate that maximizes lifetime reproductive success [2], [3], [11]. Between the sexes, the amount of energy invested in gametes by males and females is often assumed to differ, since females aim to maximize offspring survival, whereas males aim to inseminate as many females as possible [2], [3]. However, little consideration has been given to energetic limitations on the production of gamete biomass that may be imposed through constraints on whole-organism metabolism. Moreover, broad-scale interspecific comparisons of the energetic investment in gametes are rare for females and almost non-existent for males (but see [9], [12]).

Here we present a broad-scale comparative study that quantifies two key features of energetic investment in gametes by males and females for diverse species (i.e. invertebrates, reptiles, amphibians, fishes, birds, and mammals) that vary tremendously in their life histories. First, we compare the biomass allocation to gonads in males and females across a broad range of body sizes and assess differences in allocation among taxonomic groups. Like other organs, we expect gonad mass to scale approximately linearly with body mass [13], [14] and for variation about the relationship to be explained by differences in sperm competition among males and differences in clutch size among females. Second, we compare rates of production of gamete biomass in males and females across a broad range of body masses. We hypothesize that, like other rates of biomass production (e.g. growth rate, [15], [16]), the production of gamete biomass should occur at a rate proportional to whole-organism metabolic rate. This presumes that the production of gamete biomass is a function of both gonad mass and gonad metabolic rate and that gonad metabolic rate is proportional to whole-organism metabolic rate. Thus, we expect gamete biomass production rates to scale as a power law with body mass with an exponent of about , as is often observed for whole-organism metabolic rate [16], [17], [18], [19], but see [20], [21], [22]. Since males must produce seminal fluid in addition to sperm, we also assess male investment in ejaculate biomass production (i.e. gametes + seminal fluid). We then quantify the amount of energy devoted to egg, sperm, and ejaculate biomass production relative to basal metabolic rate. Finally, we consider whether residual sperm or egg biomass production rates are related to residual gonad mass. In the case of sperm, a positive relationship between residual sperm biomass production rates and residual testes mass would be consistent with sperm competition theory.

(A) The logarithm of temperature-corrected daily sperm biomass production rates (W; diamonds, dashed black line) and the logarithm of daily egg biomass production rate (W; circles, solid line) versus the logarithm of body mass (g). The relationship between metabolism and body mass for ectotherms at 20C [13] is plotted for comparative purposes (dashed orange line). (B) Residual daily gamete biomass production rates (from a log-log plot of daily gamete biomass production rates versus body mass) versus residual gonad mass (from a log-log plot of gonad mass versus body mass) males (diamonds, dashed line) and females (circles).

Our results provide insights regarding the investment by males and females in gonad and gamete biomass. Both within and between the sexes, investment in gonad biomass was quite similar across species. This is reflected in the similarity in both the slopes and intercepts of the scaling relationships of testes and ovary mass with body mass. With respect to gamete biomass production, the story appears to be quite different. Within each sex, the production of gamete biomass scaled sub-linearly with body mass across species in about the same way as whole-organism metabolic rate. However, between the sexes, rates of gamete biomass production were two to four orders of magnitude higher in females. This presents an interesting question for future research as it suggests that mass-specific rates of gamete biomass production, and perhaps mass-specific rates of metabolism in general, were much higher in ovaries than testes. 041b061a72