28 Evolutionary Primate genetics

Ms. Sangeeta Dey and Prof. A.K. Kapoor

Contents

1. Introduction

2. Characteristics of the primates

3. Classification of order primates

4. Evolutionary primate genetics 4.1 Blood groups

4.1.1. The ABO Blood Group System

4.1.2. The Rh Blood Group System

4.2 Chromosomes

4.3 Deoxyribonucleic Acid (DNA)

5. Summary

Learning Objectives:

To describe who are primates and their classification
To understand the characteristics of primates
To describe ABO Blood group system of primates
To know about chromosome number changes among primates
To understand impact of evolution for the evolutionary primate genetics
To explore the importance of genes, DNA and chromosome in primate history

1. Introduction

Primates are mammals that separated from a primitive group of mammals perhaps the small, primitive insect –eating mammals, the Insectivores some 60 million years ago. Mammals are one of the five taxa of vertebrate animals. Vertebrates are animals with vertebral column; this includes not only mammals but fish, amphibians, reptiles and birds. Mammals are characterized as warm blooded vertebrates having a advanced reproductive system, mammary glands and fur or hair somewhere on their bodies.

It is difficult to define primate as a group since they lack a single characteristics that separates them from other mammalian orders as for example, the gnawing incisor teeth of rodents, the specialized limbs and the reduced digits of horses, cows, deers and pigs or the wings of bats. Because of this, most authors have looked for evolutionary trends or total morphological patterns when attempting to define and classify primates. These trends and grades of morphological organization of non-human primates have usually been compared with those of human organization. This is satisfactory for certain type of studies but a more instructive approach for understanding primate evolution might be gained by comparing primates with other animals that have become adapted to similar life styles.

The first major definition of the primates was published by the British anatomist St. George Mivart in 1873. He wrote that primate as an order are “unguilate, claviculate placental mammals, with orbits encircled by bones; three kinds of teeth, at least at one time of their life; brain always with a posterior lobe and a calcarine fissure; the innermost digit of at least one pair of extremities opposable; hallux with a flat nail or none; a well developed cecum; penis pendulous; testes scrotal; always two pectoral mammae.”

The current definition of primates is as follows: primates have retained a generalized skeleton, with five fingers and toes or pentadactyly. The innermost digit on the hand, the thumb and foot, the big toe usually has enhanced mobility and there is a tendency for nails to replace claws on at least some of the digits. There are also touch pads possessing finger and toe prints at the ends of the digits. The orbits are enriched with bones, that is, there is postorbital bar present so that the eye is completely surrounded by bone. A characteristic of all primates is that the middle ear chamber, or auditory bulla, is formed from the petrous portion of the temporal bone. There is retention of the clavicle in all primates which enhances the mobility of the shoulder joint.

2. Characteristics of the Primates

The Primates bear flat nails upon their digits. This makes the grasping function of the hands and feet easier.
The presence of well developed clavicle is a characteristic feature of the primates.
The orbits are completely surrounded by bony rims.
The limbs are prehensile and adapted for arboreal life. This was probably an earliest mammalian characteristic.
Either the thumb or the great toe or both are opposable. This possibility, which is an ancient mammalian feature, has been retained by the primates during their long arboreal life.
The teeth are adapted for mixed food-vegetation and animal. Three kinds of teeth are present in the primate, atleast at one time of its life. The teeth are differentiated into incisors, canines and molars to serve different functions of cutting, holding and grinding, etc.
In the primates, the mammae are two in number and they are pectoral in position. The nursing habits of the mother determine the position of the mammary glands.
The development of the brains of the primates shows wide variation. But a calcarine fissure and a posterior lobe is always present in the primate brain.
The primate possess pendulous penis. The character is shown by some other mammals also.
They have a well developed caecum that is a sac connected with the intestine of an animal.
The stomach is simple.
The femur has never third trochanter.
The testes are descended into the scrotum. Of course, it is present in many other agile and active mammals also.

3. Classification of Order – Primates

4. Evolutionary primate genetics

4.1. Blood groups

Landsteiner’s discovery in 1901 of blood groups in humans initiated what turned out to be one of the most active and important research areas in clinical medicine – human genetics-and, today, organ transplants. As early as 1925, it was known that the red blood cells of non-human primates had antigens – a substance that stimulate the production of antibodies similar to those of humans. Then in 1940, landsteiner and weiner discovered the Rh factor in rhesus monkeys, one of the most important discoveries in the history of blood group studies in primates. This discovery of a monkey-type antigen in humans blood was a great impetus to study the blood of other non-human primates using other human antigens. Later, antisera were made from the red cells of non-human primates themselves and then used to ascertain the specificities of the blood of other non-human primates. The former are known as human type blood groups, while the latter are simian type blood groups. Information regarding the blood groups of non human primates is much more limited than it is for humans. At present there are several separate blood group systems under study in non-human primates, such as the well known ABO, M-N, Rh, Lewis, and Duffy. There are blood group data for all the anthropoid apes, some species of Old World monkeys, very few species of New World monkeys, and only a few prosimians. The ABO blood group is well documented in non human primates.

4.1.1. ABO Blood group system

The ABO group was the first blood group system discovered in humans by Landsteiner in 1901. It has been identified and found in all primates that have been examined. Primates have four blood groups: A, B, O, and AB (Table I).

Table I – The Four ABO Blood Groups of Human and Non-human Primates.

There are two aggultinable antigens on the red cells, A and B, and two anti-A and anti-B substances present in the sera which determine an animals blood type. Thus, red cells of a group A animal contain A but not B aggutinogen while the serum has anti-B but not anti-A aggulutinins. If a transfusion is necessary it should be from an individual whose blood cells do not become aggutinated when mixed with the blood of the recipient. In humans, persons with group O are considered universal donors while persons with the AB group are universal recipients (Table – II). These substances are present not only in primate blood but in their saliva as well, since the ability to secrete the A, B, And O antigens in the saliva is inherited as a dominant character. Also, since A and B substances are not normally detectable on the red cells of primates, other than in hominoids, saliva is used for identifying the ABO groups in other primates.

The ABO blood group frequencies for the anthropoid apes are presented in Table – III. The two species of gorilla and the simang have only B, while the pygmy chimpanzee has only the A group. These three taxa are monomorphic (one form) with respect to the ABO system, while the other apes are polymorphic (many forms). It is the interesting to note that the common chimpanzee is the only great ape having the O blood group.

Besides the obvious importance of blood groups in clinical situations such as blood transfusions and cases of materno-fetal incompatibility, they also have been of value in estimating times of evolutionary divergence, as well as in taxonomic classifications and as genetic markers. For example the fact that the ABO antigens are present in all primates studies to date would suggest that ABO polymorphism was present in the primate lineage prior to the separation of the Old and New World monkeys. Another important contribution has been in the study of hybridization of monkey populations living in overlapping geographical areas.

Table III – The ABO Blood Groups of the Lesser and Great Apes

4.1.2. Rh Blood group system

The Rh system is of interest to primate biologists because of its importance in evolutionary genetics and its practical implications for understanding the Rh factor on human red cells.

Rh factor is originally found in the rhesus monkey thus the Rh symbol refers to the rhesus monkey. In humans there is a condition known as haemolytic disease of the newborn (erythroblastosis fetalis) which causes the oxygen – carrying red blood cells of the fetus to be destroyed. The result is usually death of the fetus. This rather rare situation (about 85 percent of humans are Rh positive while 15 percent are Rh negative) occurs when the parents have incompatible Rh factors. For example, if the mother in Rh negative and the father is Rh positive, then the fetus is Rh positive. The haemolytic disease is due to the reaction of the antibodies stimulated within an Rh-negative mother by the Rh – positive cells of the fetus. In subsequent pregnancies, these anti-Rh-positive antibodies from the mother enter the fetal circulation and destroy the fetal cells, resulting in a severe anemia. It is interesting to note that among the anthropoid apes, only the red cells of the orang-utan fail to react (no homologues) with the human Rh factor.

4.2. Chromosomes

Chromosomes are coloured bodies present in each cell of an organism and carry the genetic information or DNA which is arranged in each chromosome as genes. The study of chromosomes, known as cytogenetics, has become as important branch of molecular biology. Chromosomes appear as paired structures consisting of two identical chromatids joined to each other at the primary constriction by the centromere and have different morphological characteristics that can be used in their identification. A chromosome is metameric if the centromere is near the middle of the chromatids. If the centromere is near the end of the chromatids, it is acrocentric. If the centromere is at the end of the chromatids, it is telocentric. When the centromere is between the middle and terminal positions, it is subterminal. Chromosome are counted and studies during the cell division especially during metaphase for it is during this time that the chromosomes separate and become most prominent as they line up along the plane of cell division.

The number and type of chromosomes in an organism’s cell constitute its karyotype. A specific karyotype is characteristic for a species – for example, humans have 46 chromosomes (known as the diploid or 2N number) consisting of 23 pairs of chromosomes, of which, 22 pair are autosomes and one pair are sex cells (XX, female or XY, male). The number of chromosomes present in the human germ cells is one – half the diploid number, or 23, and is known as the haploid (N) number of chromosomes. Karyotypes vary a great deal among living primates (2N ranges from 20 to 80), which indicates much selection during the course of primate evolutionary history.

Changes in the number or shape of a chromosome (chromosomal mutation) result in rearrangement of genetic material, which is often fatal or deleterious to the organism. Chromosomal rearrangements include loss of chromosome material (deletion), duplication of material, turning pieces of chromosomes upside down (inversion), and translocation (exchange of material between chromosomes. Since such changes are rare, and a given number and form of chromosomes remain stable for a particular species, there is probably strong selection pressure against such events. However, these events do occur in populations, and if they take place over many generations the result is chromosome evolution. In other words, these rearrangements constitute the raw material for karyotype evolution which may result in different species.

The number of chromosomes (2N) varies among primates. For example, some species of Lepilemur have the lowest number of chromosomes recorded among primates (2N = 20), while the specialized little tarsiers have the most (2N = 80). Among the New World monkey chromosome number ranges from 20 to 62. The 2N chromosome number of Old World monkeys varies from 42 to 72. The macaques and baboons, all have a 2N of 42.

The chromosome number ranges from 44 to 52 in hominids. The highly derived chromosomes of gibbons (2N = 38 to 52) have undergone rapid and extensive changes since separating from the other hominids. Gibbons have mostly metameric and submetameric chromosomes, although symphalangus has a single pair of acrometric chromosomes. The great apes have 2N= 48 chromosomes, and all have three types of chromosomes. The human 2N = 46 has resulted from the fusion of two acrocentric chromosomes to form the number 2 chromosome of humans. Humans have all three types of chromosomes. The sex chromosomes are similar (X and Y) in the great apes and humans – that is X chromosome is submetacentric and the Y chromosome is acrocentric.

The 2N number of primate chromosome numbers is represented by the prosimians (20 to 80), while the great ape and humans have the smallest range of chromosomes (46 – 48). Different species may have the same number of chromosomes, but their form may be quite different, so the characterization of a species’ karyotype must consider both the number and form of the chromosomes.

4.3. Deoxyribonucleic acid (DNA)

One of the most important biological discoveries of the twentieth century was the identification of the structure and function of deoxyribonucleic acid, DNA. This material, which with protein makes up the majority of the cell nucleus, contain the genetic message that enables the cell to replace itself, as well as providing information necessary for the cell’s many other functions. The essential process of life, therefore, is replication, and it is important that the copying process proceeds as accurately as possible. However, mistakes (mutations) do occur in the copying process, but these are usually harmful to the organism. Occasionally a mutation happens to be beneficial to the organism and selection occurs and the mutation survives and spreads through the population. The result is evolution by natural selection which through millions of years has produced the diversity of life present today on earth.

Advances in DNA technology have made it possible to sequence genes and compare them with genes from other species. Also, fragments of DNA from two different species can be combined into a hybrid segment, a process called DNA hybridization. There are four types of bases in DNA; adenine, guanine, cytosine and thymine. The number of matches and mismatches between the bases on the hybrid strands of each DNA fragment allows the determination of the degree of similarity between the two species at the DNA level. Such studies have provided information on the degree of genetic relationships among primates and have demonstrated that the DNA of chimpanzees, gorillas and humans differs only about 2 percent. This means that when strands of DNA from any two of these animals are combined about 98 percent of the bases match. Humans differ from orang-utans by about 4 percent and from baboons by about 8 percent. This similarity of DNA sequences among primate taxa indicates their degree of biological evolutionary relationship.

5. Summary

Primates are mammals that separated from a primitive group of mammals perhaps the small, primitive insect –eating mammals, the Insectivores some 60 million years ago. Landsteiner’s discovery in 1901 of blood groups in humans initiated what turned out to be one of the most active and important research areas in clinical medicine – human genetics. The Study of human genetics through blood groups, chromosomes, DNA and their similarity with other primates has led the evolutionary history more predictable.

ABO Blood group system has obvious importance in clinical situations such as blood transfusions and cases of materno-fetal incompatibility. They also have been of value in estimating times of evolutionary divergence, as well as in taxonomic classifications and as genetic markers. For example the fact that the ABO antigens are present in all primates studies to date would suggest that ABO polymorphism was present in the primate lineage prior to the separation of the Old and New World monkeys.

Different species may have the same number of chromosomes, but their form may be quite different, so the characterization of a species’ karyotype must consider both the number and form of the chromosomes.

One of the most important biological discoveries of the twentieth century was the identification of the structure and function of deoxyribonucleic acid, DNA. The essential process of life is replication that is the copying process. Any mistake (mutation) in copying process results in change in genetic makeup of the organism. Sometimes, these mutations are beneficial to the organism and selection occurs and the mutation survives and spreads through the population. The result is evolution by natural selection which through millions of years has produced the diversity of life present today on earth.

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