《生物学》课程参考书籍文献（英文版，Chapter 11 - Chapter 19）Part IV Reproduction and Heredity、Part V Molecular Genetics

Part IV Reproduction and Heredity ① Why Do Some Genes Maintain More Than One Common Allele in a Population? ∽ These bacterial cells are dividing. As the population grows, plant was supposed to look like: small plant with green gene variants arise by mutation. Do the new variants persist,or ds. But if all pea p nts are they eliminated by natural selection? ble to sort out th A Muller's classical model thus makes a very straightfor- les that ward prediction: in nature, most populations of asexual organisms should be genetically uniform most of the time. However, this is not at all what is observed. Natural popu- uding asexua nes like bacteria appear to have ise. Selection favors one form at a particular plac fferer at a differer varying sel crown onment, an en- each to b tion tween them G call balance by by natural selection, an d either wins or loses. tualism) or favors one (what a biologist calls commensal- There are no ties. One version of the gene becomes univer- sal in the population, and the other geneinated. popula ism). In essence, cooperation would be counterbalancing the effects of competition 205

205 Why Do Some Genes Maintain More Than One Common Allele in a Population? When Mendel did his crosses of pea plants, he knew what a pea plant was supposed to look like: a small plant with green leaves, purple flowers, and smooth seeds. But if all pea plants were like that, he would never have been able to sort out the rules of heredity—in a cross of green peas with green peas, there would have been no visible differences to reveal the 3:1 pattern of gene segregation. The variant alleles that Mendel employed in his studies—yellow leaves, white flow￾ers, wrinkled seeds—were rare “accidents” maintained in seed collections for their novelty. In nature, such unusual kinds of peas had never been encountered by Mendel. By the time Mendel’s work was rediscovered in 1900, Darwin had provided a ready explanation of why alterna￾tive alleles seemed to be rare in natural populations. Nat￾ural selection was simply scouring the population, cleansing it in each generation of less fit alternatives. While recombi￾nation can complicate the process in interesting ways among sexual organisms like peas, asexual organisms like bacteria were predicted to be very sensitive to the effects of selection. Left to do its work, natural selection should crown as winner in bacterial population the best allele of each gene, producing a uniform population. Why do populations contain variants at all? In 1932 the famous geneticist Herman Muller formulated what has come to be called the “classical model,” explaining gene variation in natural populations of asexual organisms as a temporary, transient condition, new variations arising by random muta￾tion only to be established or eliminated by selection. Except for the brief periods when populations are undergoing this periodic cleansing, they should remain genetically uniform. The removal of variants was proposed to be a very straightforward process. During the periodic cleansing pe￾riods envisioned by Muller, his classical model operates under a “competitive exclusion” principle first proposed by Gause: whenever a new variant appears, it is weighed in the balance by natural selection, and either wins or loses. There are no ties. One version of the gene becomes univer￾sal in the population, and the other is eliminated. Muller’s classical model thus makes a very straightfor￾ward prediction: in nature, most populations of asexual organisms should be genetically uniform most of the time. However, this is not at all what is observed. Natural popu￾lations of most species, including asexual ones like bacteria, appear to have lots of common variants—they are said to be “polymorphic.” So where are all of these variants coming from? Varia￾tion in the environment, either spatial or temporal, can be used to explain how some polymorphisms arise. Selection favors one form at a particular place and time, a different form at a different place or time. In a nutshell, varying selection can encourage polymorphism. Is that all there is to it? Is it really impossible for more than one variant to become common in a population, if the population lives in a constant uniform environment, an en￾vironment that does not vary from one place to another or from one time to another? Theory says so. Biologists that study microbial communities have begun to report that bacteria are not aware of Muller’s theory. Bacterial cultures started from a single cell living in simple unstructured environments rapidly become polymorphic. There is a way to reconcile theory and experiment. Per￾haps the variant individuals in the population are interact￾ing with one another. Muller’s theory assumes that every individual undergoes an independent trial by selection. But what if that’s not so? What if different kinds of individuals help each other out? Stable coexistence of variants in a population might be possible if interactions between them contribute to the welfare of both (what a biologist calls mu￾tualism) or favors one (what a biologist calls commensal￾ism). In essence, cooperation would be counterbalancing the effects of competition. Part .04 µm IV Reproduction and Heredity These bacterial cells are dividing. As the population grows, gene variants arise by mutation. Do the new variants persist, or are they eliminated by natural selection? Real People Doing Real Science

Acetate media E 0.06 ●cV101 strain 0.04 90.02 (a) (b) Maintaining stable polymorphism (a)Three new strains emerge in culture and are maintained. (b)Two strains are grown on media containing acetate. The strain CV103 was found to excrete acetate, while the strain CV101 was found to thrive in media with acetate as the sole source of carbon. Population growth is measured by an increase in the turbidity of the liquid medium; turbidity is measured as an increase in light absorbance at a wavelength of 420 nm(A420 nm) The Experiment The results To investigate this intriguing possibility, Julian Adams Three distinct variants were detected in the 773-generation and co-workers at the University of Michigan set out to E. coli, each being maintained at stable levels in the contin- see if polymorphism for metabolic abilities would de- uously growing culture. Clearly polymorphism can appear velop spontaneously in bacteria growing in a uniform within an initially uniform bacterial population growing in environment a simple homogeneous environment. For a bacterial subject they chose Escherichia coli When mixed together and allowed to compete, one (E coli), a widely studied bacterium whose growth under strain does not drive the other two to extinction, as theory laboratory conditions is well understood. Cultures of had predicted. Instead, the three new strains, CV101 Escherichia coli can be maintained in chemostat culture CV103, and CV116, all persist(see graph a above). for many hundreds of generations. A chemostat is a large The three strains were then analyzed to see how they ontainer holding liquid culture medium. A little bit of differed. CV103 exhibited the highest rate of glucose up- the liquid is continuously removed, and an equal amount take and produced the most acetate(an end product of glu- of fresh culture medium added to replace what leaves. cose aerobic fermentation). Is this difference important? The growth of the E coli culture is limited by the amount To see, the CV103 strain was co-cultured with CV101 of glucose remaining in the culture medium to feed the They maintained stable growth levels, which indicated that the contribution of the third strain. CV116 was not re Researchers inoculated a glucose-limited chemostat cul- quired to maintain their growth. ture media with the E. coli strain JA122, and maintained the What is the difference between cv1o1 and cv1033 continuous culture for 773 generations. A sample was CV101 could grow in culture filtrate of CV103 but in the taken from the chemostat after 773 generations and an reverse situation, CV103 could not grow. This indicates lyzed for the presence of new strains of E. coli. Any varia- that CV103 secretes a substance upon which CV101 can tion among the cells in the sample would indicate that grow. Is CV101 utilizing the acetate produced by CV103 polymorphism had arisen as its carbon source? To detect metabolic variation within the sample of To test this possibility, CV101 and CV103 were grown growing cells, Adams's team analyzed the rate of glucose together in media with acetate as the only carbon source uptake and the concentration of acetate, among other The results from this experiment are shown in graph b variables. By examining such biochemical parameters, above and indicate that Cv101 thrives on an acetate carbon the researchers could determine if the different strains source, while Cv103 does not and requires an additional were filling different metabolic "niches"that is, using carbon source such as glucose the metabolic environment in different ways. Metabolic These results indicate that two of the strains are main- niches were characterized by looking at the normal prod- tained in polymorphism at stable levels because they have ucts of aerobic fermentation, acetate and glycerol, which evolved different adaptations that allow them to coexist by appear in the growth medium as a by-product of E. coli filling different niches. One strain(Cv101)is maintained metabolism in the population because it is able to use a metabolic by To further classify the strains, batch cultures containing product released by another strain(CV103) wo strains were established to analyze interactions be To explore this experiment further, tween the two groups go to the virtual lab at mhhe. com/raven/vlab4. mhtml

The Experiment To investigate this intriguing possibility, Julian Adams and co-workers at the University of Michigan set out to see if polymorphism for metabolic abilities would de￾velop spontaneously in bacteria growing in a uniform environment. For a bacterial subject they chose Escherichia coli (E. coli), a widely studied bacterium whose growth under laboratory conditions is well understood. Cultures of Escherichia coli can be maintained in chemostat culture for many hundreds of generations. A chemostat is a large container holding liquid culture medium. A little bit of the liquid is continuously removed, and an equal amount of fresh culture medium added to replace what leaves. The growth of the E. coli culture is limited by the amount of glucose remaining in the culture medium to feed the growing cells. Researchers inoculated a glucose-limited chemostat cul￾ture media with the E. coli strain JA122, and maintained the continuous culture for 773 generations. A sample was taken from the chemostat after 773 generations and ana￾lyzed for the presence of new strains of E. coli. Any varia￾tion among the cells in the sample would indicate that polymorphism had arisen. To detect metabolic variation within the sample of growing cells, Adams’s team analyzed the rate of glucose uptake and the concentration of acetate, among other variables. By examining such biochemical parameters, the researchers could determine if the different strains were filling different metabolic “niches”—that is, using the metabolic environment in different ways. Metabolic niches were characterized by looking at the normal prod￾ucts of aerobic fermentation, acetate and glycerol, which appear in the growth medium as a by-product of E. coli metabolism. To further classify the strains, batch cultures containing two strains were established to analyze interactions be￾tween the two groups. The Results Three distinct variants were detected in the 773-generation E. coli, each being maintained at stable levels in the contin￾uously growing culture. Clearly polymorphism can appear within an initially uniform bacterial population growing in a simple homogeneous environment. When mixed together and allowed to compete, one strain does not drive the other two to extinction, as theory had predicted. Instead, the three new strains, CV101, CV103, and CV116, all persist (see graph a above). The three strains were then analyzed to see how they differed. CV103 exhibited the highest rate of glucose up￾take and produced the most acetate (an end product of glu￾cose aerobic fermentation). Is this difference important? To see, the CV103 strain was co-cultured with CV101. They maintained stable growth levels, which indicated that the contribution of the third strain, CV116, was not re￾quired to maintain their growth. What is the difference between CV101 and CV103? CV101 could grow in culture filtrate of CV103 but in the reverse situation, CV103 could not grow. This indicates that CV103 secretes a substance upon which CV101 can grow. Is CV101 utilizing the acetate produced by CV103 as its carbon source? To test this possibility, CV101 and CV103 were grown together in media with acetate as the only carbon source. The results from this experiment are shown in graph b above and indicate that CV101 thrives on an acetate carbon source, while CV103 does not and requires an additional carbon source such as glucose. These results indicate that two of the strains are main￾tained in polymorphism at stable levels because they have evolved different adaptations that allow them to coexist by filling different niches. One strain (CV101) is maintained in the population because it is able to use a metabolic by￾product released by another strain (CV103). Generations 0.4 0.6 0.8 Frequency in population Population growth (A420 1.0 nm) 10 20 30 0.2 0.0 Time (hours) 0.02 0.04 0.06 10 20 30 40 0.00 CV101 strain CV103 strain CV116 strain (a) (b) CV101 strain Acetate media CV103 strain Maintaining stable polymorphism. (a) Three new strains emerge in culture and are maintained. (b) Two strains are grown on media containing acetate. The strain CV103 was found to excrete acetate, while the strain CV101 was found to thrive in media with acetate as the sole source of carbon. Population growth is measured by an increase in the turbidity of the liquid medium; turbidity is measured as an increase in light absorbance at a wavelength of 420 nm (A420 nm). To explore this experiment further, go to the Virtual Lab at www.mhhe.com/raven6/vlab4.mhtml

How Cells divide Concept outline 11.1 Bacteria divide far more simply than do Cell Division in Prokaryotes. Bacterial cells divide by pitting in two 11.2 Chromosomes are highly ordered structures. Discovery of Chromosomes. All eukaryotic cells contain omosomes but different numbers of chromosomes The Structure of Eukaryotic Chromos play an important role in packaging DNA in chromosome 1.3 Mitosis is a key phase of the cell cycle. Phases of the Cell Cycle. The cell cycle cor growth phases, a nuclear division phase, and a division Interphase: Preparing for Mitosis. cell grows, replicates its DNA, and prepares for cell FIGURE 11.1 Mitosis. In prophase, the chromosomes condense and Cell division in bacteria. It,s hard to imagine fecal coliform microtubules attach sister chromosomes to opposite poles acteria as beautiful. but here is escherichia coli inhabitant of the of the cell In metaphase, chromosomes align along the large intestine and the biotechnology lab, spectacularly caught in center of the cell In anaphase, the chromosomes separate the act of fission in telophase the spindle dissipates and the nuclear envelope reforms Cytokinesis. In cytokinesis, the cytoplasm separates into Ai ces of organisms--bacteria, alligators, the weeds two roughly equal halves. n a lawn--grow and reproduce. From the smallest of 11. 4 The cell cycle is carefully controlled. creatures to the largest, all species produce offspring like themselves and pass on the hereditary information that General Strategy of Cell Cycle Control. At three points akes them what they are. In this chapter, we begin our in the cell cycle, feedback from the cell determines whether consideration of heredity with an examination of how cells reproduce(figure 11.1). The mechanism of cell reproduc Molecular Mechanisms of Cell Cycle Control. Special tion and its biological consequences have changed signifi Cancer and the Control of Cell Proliferation. Cancer cantly during the evolution of life on earth results from damage to genes encoding proteins that regulate the cell division cycle 207

207 11 How Cells Divide Concept Outline 11.1 Bacteria divide far more simply than do eukaryotes. Cell Division in Prokaryotes. Bacterial cells divide by splitting in two. 11.2 Chromosomes are highly ordered structures. Discovery of Chromosomes. All eukaryotic cells contain chromosomes, but different organisms possess differing numbers of chromosomes. The Structure of Eukaryotic Chromosomes. Proteins play an important role in packaging DNA in chromosomes. 11.3 Mitosis is a key phase of the cell cycle. Phases of the Cell Cycle. The cell cycle consists of three growth phases, a nuclear division phase, and a cytoplasmic division stage. Interphase: Preparing for Mitosis. In interphase, the cell grows, replicates its DNA, and prepares for cell division. Mitosis. In prophase, the chromosomes condense and microtubules attach sister chromosomes to opposite poles of the cell. In metaphase, chromosomes align along the center of the cell. In anaphase, the chromosomes separate; in telophase the spindle dissipates and the nuclear envelope reforms. Cytokinesis. In cytokinesis, the cytoplasm separates into two roughly equal halves. 11.4 The cell cycle is carefully controlled. General Strategy of Cell Cycle Control. At three points in the cell cycle, feedback from the cell determines whether the cycle will continue. Molecular Mechanisms of Cell Cycle Control. Special proteins regulate the “checkpoints” of the cell cycle. Cancer and the Control of Cell Proliferation. Cancer results from damage to genes encoding proteins that regulate the cell division cycle. All species of organisms—bacteria, alligators, the weeds in a lawn—grow and reproduce. From the smallest of creatures to the largest, all species produce offspring like themselves and pass on the hereditary information that makes them what they are. In this chapter, we begin our consideration of heredity with an examination of how cells reproduce (figure 11.1). The mechanism of cell reproduc￾tion and its biological consequences have changed signifi￾cantly during the evolution of life on earth. FIGURE 11.1 Cell division in bacteria. It’s hard to imagine fecal coliform bacteria as beautiful, but here is Escherichia coli, inhabitant of the large intestine and the biotechnology lab, spectacularly caught in the act of fission

11.1 Bacteria divide far more simply than do eukaryotes Cell Division in Prokaryotes In bacteria, which are prokaryotes and lack a nucleus, cell division consists of a simple procedure called binary fission (literally, "splitting in half), in which the cell divides into two equal or nearly equal halves(figure 11. 2). The genetic information, or genome, replicates early in the life of the cell. It exists as a single, circular, double-stranded DNA mole chis dna circle into the bacterial cell is a re- markable feat of packaging--fully stretched out, the dNA of a bacterium like Escherichia coli is about 500 times longer The dna circle is attached at one point to the cytoplas- mic surface of the bacterial cells plasma membrane. at a pecific site on the dna molecule called the replication ori- gin, a battery of more than 22 different proteins begins the process of copying the DNA(figure 11). When these en-FIGURE 11.2 “ daughter” genomes are attached side- by-side to the plasma two daughter cells幺巨 zymes have proceeded all the way around the circle of Fission(40,000x). Bacteria divide by a process of simple cell DNA, the cell possesses two copies of the genome. These fission. Note the newly formed plas membrane The growth of a bacterial cell to about twice its initial ize induces the onset of cell division a wealth of recent ev- idence suggests that the two daughter chromosomes are ac- ively partitioned during this process. As this process pro- cells are much larger than bacteria, and their genomes con- ceeds, the cell lays down new plasma membrane and cell tain much more DNA. Eukaryotic DNA is contained in a wall materials in the zone between the attachment sites of number of linear chromosomes, whose organization is much the two daughter genomes. A new plasma membrane grows more complex than that of the single, circular DNA mole- between the genomes; eventually, it reaches all the way into cules in bacteria. In chromosomes, DNA forms a complex the center of the cell, dividing it in two. Because the mem- with packaging proteins called histones and is wound into brane forms between the two genomes, each new cell is as- tightly condensed coils sured of retaining one of the genomes. Finally, a new cel wall forms around the new membrane Bacteria divide by binary The evolution of the eukaryotes introduced several addi middle of the cell. An activ hat one genome will end d up in ca Fission begins in the tioning process ensures tional factors into the process of cell division. Eukaryotic each daughter cell ⑧8 Replication FIGURE 11.3 How bacterial DNA repl The replication of the circular DNA molecule(blue)that constitutes the genome of a bacterium begins at a single site, called the rep origin. The replication enzymes move out in both directions from that site and make copies(red) of each strand in the dna duplex the enzymes meet on the far side of the molecule, replication is complete 208 Part IV Reproduction and Heredity

cells are much larger than bacteria, and their genomes con￾tain much more DNA. Eukaryotic DNA is contained in a number of linear chromosomes, whose organization is much more complex than that of the single, circular DNA mole￾cules in bacteria. In chromosomes, DNA forms a complex with packaging proteins called histones and is wound into tightly condensed coils. Bacteria divide by binary fission. Fission begins in the middle of the cell. An active partitioning process ensures that one genome will end up in each daughter cell. 208 Part IV Reproduction and Heredity Cell Division in Prokaryotes In bacteria, which are prokaryotes and lack a nucleus, cell division consists of a simple procedure called binary fission (literally, “splitting in half”), in which the cell divides into two equal or nearly equal halves (figure 11.2). The genetic information, or genome, replicates early in the life of the cell. It exists as a single, circular, double-stranded DNA mole￾cule. Fitting this DNA circle into the bacterial cell is a re￾markable feat of packaging—fully stretched out, the DNA of a bacterium like Escherichia coli is about 500 times longer than the cell itself. The DNA circle is attached at one point to the cytoplas￾mic surface of the bacterial cell’s plasma membrane. At a specific site on the DNA molecule called the replication ori￾gin, a battery of more than 22 different proteins begins the process of copying the DNA (figure 11.3). When these en￾zymes have proceeded all the way around the circle of DNA, the cell possesses two copies of the genome. These “daughter” genomes are attached side-by-side to the plasma membrane. The growth of a bacterial cell to about twice its initial size induces the onset of cell division. A wealth of recent ev￾idence suggests that the two daughter chromosomes are ac￾tively partitioned during this process. As this process pro￾ceeds, the cell lays down new plasma membrane and cell wall materials in the zone between the attachment sites of the two daughter genomes. A new plasma membrane grows between the genomes; eventually, it reaches all the way into the center of the cell, dividing it in two. Because the mem￾brane forms between the two genomes, each new cell is as￾sured of retaining one of the genomes. Finally, a new cell wall forms around the new membrane. The evolution of the eukaryotes introduced several addi￾tional factors into the process of cell division. Eukaryotic 11.1 Bacteria divide far more simply than do eukaryotes. FIGURE 11.2 Fission (40,000). Bacteria divide by a process of simple cell fission. Note the newly formed plasma membrane between the two daughter cells. Replication origin FIGURE 11.3 How bacterial DNA replicates. The replication of the circular DNA molecule (blue) that constitutes the genome of a bacterium begins at a single site, called the replication origin. The replication enzymes move out in both directions from that site and make copies (red) of each strand in the DNA duplex. When the enzymes meet on the far side of the molecule, replication is complete.

11.2 Chromosomes are highly ordered structures Discovery of Chromosomes Chromosomes were first observed by the german embryol ogist Walther Fleming in 1882, while he was examining the rapidly dividing cells of salamander larvae. When Fleming looked at the cells through what would now be a rather primitive light microscope, he saw minute threads within their nuclei that appeared to be dividing lengthwise. Flem ing called their division mitosis, based on the greek word anng“t Chromosome number Since their initial discovery, chromosomes have been found in the cells of all eukaryotes examined. Their number may ary enormously from one species to another. a few kinds of organisms--such as the Australian ant My armeria, the pla Haplopappus gracilis, a relative of the sunflower that grows in FIGURE 11.4 North American deserts; and the fungus penicilliumn--have Human chromosomes. This photograph(950x) shows human only 1 pair of chromosomes, while some ferns have more chromosomes as they appear immediately before nuclear division than 500 pairs(table 11.1). Most eukaryotes have between Each DNA molecule has already replicated, forming identical 10 and 50 chromosomes in their body cells opies held together by a constriction called the centromere. Human cells each have 46 chromosomes. consist ing of 23 nearly identical pairs(figure 11.4). Each of these 46 chromosomes contains hundreds or thou- trisomy is fatal, and even in those few cases, sands of genes that play important roles in determin- problems result. Individuals with an extra copy of the ing how a person's body develops and functions. For very small chromosome 21, for example, develop this reason, possession of all the chromosomes is es more slowly than normal and are mentally retarded, a sential to survival. humans miss even one condition called Down syndrome mosome, a condition called monosomy, do not sur- vive embryonic development in most cases. Nor does All eukaryotic cells store their hereditary information in c. e human embryo develop proper. on called tri- chromosomes, but different kinds of organisms utilize very different numbers of chromosomes to store this somy. For all but a few of the smallest chromosomes, information Table 11.1 Chromosome Number in Selected Eukaryotes Total Number of Total Number of Total Number of Chromosomes Group Chromosomes Group Chromosomes FUNGI PLANTS VERTEBRATES Neurospora(haploid) Haplopappus graci veast Garden pea 14 Mor INSECTS Corn Bread wheat Human Drosophila 6826 48 Horsetail Ho Adder s tongue fern 1262 Chicken Chapter 11 How Cells Divide 209

Discovery of Chromosomes Chromosomes were first observed by the German embryol￾ogist Walther Fleming in 1882, while he was examining the rapidly dividing cells of salamander larvae. When Fleming looked at the cells through what would now be a rather primitive light microscope, he saw minute threads within their nuclei that appeared to be dividing lengthwise. Flem￾ing called their division mitosis, based on the Greek word mitos, meaning “thread.” Chromosome Number Since their initial discovery, chromosomes have been found in the cells of all eukaryotes examined. Their number may vary enormously from one species to another. A few kinds of organisms—such as the Australian ant Myrmecia, the plant Haplopappus gracilis, a relative of the sunflower that grows in North American deserts; and the fungus Penicillium—have only 1 pair of chromosomes, while some ferns have more than 500 pairs (table 11.1). Most eukaryotes have between 10 and 50 chromosomes in their body cells. Human cells each have 46 chromosomes, consist￾ing of 23 nearly identical pairs (figure 11.4). Each of these 46 chromosomes contains hundreds or thou￾sands of genes that play important roles in determin￾ing how a person’s body develops and functions. For this reason, possession of all the chromosomes is es￾sential to survival. Humans missing even one chro￾mosome, a condition called monosomy, do not sur￾vive embryonic development in most cases. Nor does the human embryo develop properly with an extra copy of any one chromosome, a condition called tri￾somy. For all but a few of the smallest chromosomes, trisomy is fatal, and even in those few cases, serious problems result. Individuals with an extra copy of the very small chromosome 21, for example, develop more slowly than normal and are mentally retarded, a condition called Down syndrome. All eukaryotic cells store their hereditary information in chromosomes, but different kinds of organisms utilize very different numbers of chromosomes to store this information. Chapter 11 How Cells Divide 209 11.2 Chromosomes are highly ordered structures. FIGURE 11.4 Human chromosomes. This photograph (950×) shows human chromosomes as they appear immediately before nuclear division. Each DNA molecule has already replicated, forming identical copies held together by a constriction called the centromere. Table 11.1 Chromosome Number in Selected Eukaryotes Total Number of Total Number of Total Number of Group Chromosomes Group Chromosomes Group Chromosomes FUNGI Neurospora (haploid) 7 Saccharomyces (a yeast) 16 INSECTS Mosquito 6 Drosophila 8 Honeybee 32 Silkworm 56 PLANTS Haplopappus gracilis 2 Garden pea 14 Corn 20 Bread wheat 42 Sugarcane 80 Horsetail 216 Adder’s tongue fern 1262 VERTEBRATES Opossum 22 Frog 26 Mouse 40 Human 46 Chimpanzee 48 Horse 64 Chicken 78 Dog 78