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Sexual reproduction is the formation of a new individual following the union of two gametes. In humans and the majority of other eukaryotes plants as well as animals the two gametes differ in structure ("anisogamy") and are contributed by different parents. Gametes need motility to be able to meet and unite and food to nourish the developing embryo. In animals (and some plants), these two rather contrasting needs are met by anisogametes: sperm that are motile (and small) and eggs that contain food.
In multicellular organisms individual cells grow and then divide via a process called mitosis, thereby allowing the organism to grow. The organism begins as a single cell (fertilized egg) that divides successively to produce many cells, with each parent cell passing identical genetic material (two variants of each chromosome pair) to both daughter cells. Cellular division and differentiation produce and maintain a complex organism, composed of systems of tissues and organs that work together to meet the needs of the whole organism.
In sexual reproduction, chromosomes can sometimes swap sections during the process of meiosis (cell division), thereby creating new genetic combinations and thus more genetic variation. Although DNA replication is tightly regulated and remarkably accurate, errors do occur and result in mutations, which are also a source of genetic variation. Environmental factors can also cause mutations in genes, and viable mutations are inherited.
The student is expected to use a model to illustrate the role of cellular division (mitosis) and differentiation in producing and maintaining complex organisms.
Iowa Core Standard Correlation:
HS-LS1-4. Use a model to illustrate the role of cellular division (mitosis) and differentiation in producing and maintaining complex organisms.
HS-LS3-2. Make and defend a claim based on evidence that inheritable genetic variations may result from (1) new genetic combinations through meiosis, (2) viable errors occurring during replication, and/or (3) mutations caused by environmental factors.
15.7: Sexual Reproduction - Biology
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Sexual Dimorphism and Seasonal Variation of Reproductive Hormones in the Pascagoula Map Turtle, Graptemys gibbonsi
Sean P. Graham, 1 Chelsea K. Ward, 2 Jennifer Shelby Walker, 3 Sean Sterrett, 4 Mary T. Mendonça 3
1 1Department of Biology, The Pennsylvania State University, 508 Mueller Lab, University Park, Pennsyl
2 2Department of Biology, Auburn University Montgomery, 7061 Senators Dr., Montgomery, Alabama 36117
3 3Department of Biological Sciences, Auburn University, 331 Funchess Hall, Auburn, Alabama 36849 E-m
4 4Warnell School of Forestry and Natural Resources, University of Georgia, 180 E Green St., Athens, G
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The Pascagoula Map Turtle (Graptemys gibbonsi) is a narrowly endemic species found only in the Pascagoula River drainage in Mississippi. It is among the most poorly known turtle species because of research taxonomic biases and this species' relatively recent recognition as a unique taxon. A recent petition requested protective status for G. gibbonsi under the U.S. Endangered Species Act. We describe population parameters, quantitatively assess sexual dimorphism of G. gibbonsi, and document hormone secretion patterns from the Chickasawhay and Leaf rivers in Mississippi. We demonstrate a significant male-skewed sex ratio and a female-biased size dimorphism in both carapace length and height. Males showed a bimodal peak of plasma testosterone in fall and spring, consistent with the pattern shown by many other southeastern turtles with late summer-fall spermatogenesis and mating during spring and fall. Females did not show seasonal variation in estradiol secretion, an unexpected result that was possibly due to our small sample size of females, none of which were gravid when captured. Although this observation may be due to our limited capacity to sample females, given the reproductive issues reported for Graptemys flavimaculata from the same drainage (e.g., reproductive hormone abnormalities, low nesting frequency and success), this finding warrants concern and necessitates additional research. Finally, in order to put our hormone data in context, we briefly review hormone and reproductive patterns in southeastern U.S. turtles. Our review includes the timing of follicular enlargement, ovulation and nesting, clutch frequency, and estradiol cycles. The review for male turtles includes details on the spermatogenic cycle, spermiation, and the timing and frequency of testosterone peaks.
2015 by the American Society of Ichthyologists and Herpetologists
Sean P. Graham , Chelsea K. Ward , Jennifer Shelby Walker , Sean Sterrett , and Mary T. Mendonça "Sexual Dimorphism and Seasonal Variation of Reproductive Hormones in the Pascagoula Map Turtle, Graptemys gibbonsi," Copeia 2015(1), 42-50, (9 February 2015). https://doi.org/10.1643/CP-13-157
Received: 2 December 2013 Accepted: 15 September 2014 Published: 9 February 2015