Marsupial Vs Placental

Australia: The Land Where Time Began

Marsupial Vs Placental

It has long been thought that marsupials flourished in Australia because the placental mammals that dominate all other habitable continents never reached Australia. However, there is now some evidence that placentals were present in Australia in the early days of marsupial evolution, but still the marsupials prevailed. Is there something about the Australian climate, environment, or soils that favours the marsupial way of life? In the case of kangaroos and wallabies they have at least 2 adaptations that allow them to thrive in Australia's dry and erratic climate. The hopping form of locomotion is very efficient for travelling over long distances while conserving energy, as the elastic tendons of their back legs store energy derived from the previous jump by stretching, then the next jump is mostly powered by the elastic rebound of the tendon, reducing the amount of energy required for the jump. The other is their ability to have young at the 3 different stages of development at the same time. One out of the pouch but still suckling, one attached to a nipple and a fertilised ovum that remains dormant at the blastocyst stage, embryonic diapause, until required.

The 2 nipples used by the hopping young and the attached young produce different types of milk to suit the stage of development of the animal using it. This form of reproduction allows the animals to respond quickly to improved conditions and reduced populations can rebound much more quickly than if they had to wait for the next breeding season to start the process of reproduction.

In the arid interior some rock wallabies living on rocky outcrops have developed the ability to help their semi-independent young survive droughts when the vegetation is mostly dry. During easier times they get most or all the water they need from their food, but when less is available in the food the mothers can regurgitate water for their young which allows the young wallaby to continue eating the dry vegetation.

Koalas have developed the ability to eat the leaves of gum trees (Eucalypts) which are not only low in nutrition, but are packed with toxins. This ability to survive on a very widespread food source source that no other mammal can eat, and that is highly adapted to the arid climates of many parts of Australia, frees them from the pressures of competition from other mammals. Gum trees have adapted to survive drought better than most other trees, so koalas have a constant food source that also provides their water needs in dry spells when ground-dwelling animals have much more difficulty finding enough water, either free or in vegetation, to survive.

Reproductive anatomy of marsupials

The female reproductive structures of the 3 groups of living mammals are unequivocally distinct. When the foetus of a Tammar wallaby 4 days before birth is compared with a human embryo at 5 weeks of gestation, the arrangement of kidney ducts, genital ducts and gonads are the same. Kidney ducts, ureters, and both genital ducts combine into the urogenital sinus, a common tube, on its dorsal side and the future bladder on its ventral side. The ureters migrate to the ventral side to enter the bladder, in both groups, later in development, the genital ducts remaining dorsal. The ureter migration occurs in placental and marsupials, but not in monotremes. This is believed to be an adaptation for more efficient urine storage in the bladder. In monotremes, the urine discharges into the top of the sinus, passing across the sinus to enter the bladder. The exact mechanism for the this is not known.

The ureters migrate outside and below the genital ducts in placentals, including humans, but in marsupials the ureters migrate inside and above the genital ducts. In the ancestral mammals, it is believed the initial adaptation was associated mostly with excretion, the route taken to the bladder by the ureters distinguishes the placentals from the marsupials, with profound consequences for reproduction. The most obvious difference between placentals is the female reproductive tract. In marsupials there are 2 full sets of structures, 2 fallopian tubes, 2 uteruses, 2 cervixes and 2 vaginas., while in placentals, the only parts still separate are the fallopian tubes. The union of the 2 sets of reproductive structures in marsupials is prevented by the path of the ureters that pass between them, which results in 2 lateral vaginas, both of which arise from the posterior of the common urogenital sinus. The 2 vaginas loop back to the midline above the ureters, becoming partially fused.

At the onset of birth in most marsupials, a canal forms that passes from the lateral vaginas directly to the urogenital sinus, between the 2 ureters, through which the very small, immature baby passes. This pseudovaginal canal, or birth canal, reforms at each birth in almost all marsupials, the few exceptions include Macropus, the kangaroo and Tarsipes rostratus the honey possum. In these species it remains after the first birth and is called the median vagina. The 2 lateral vaginas receive sperm and the baby passes through the median vagina at birth. It has long been believed that the arrangement of the birth canal and the small size of the 2 uteri are the reasons marsupial young are so small at birth. This is yet to be proven.

Fetal membranes

As placentation evolved in mammals there was the accompanying evolution of new fetal structures beyond those present in reptiles, the authors³ saying that 'it is not surprising that parallels between marsupials and monotremes are also seen in the development and function of the fetal membranes'. In monotremes the yolk content of the egg is greatly reduced compared to that of reptiles, to the point where it is not sufficient to maintain development. Nutrients secreted by the endometrial gland are absorbed, probably being absorbed by the yolk sac, as occurs in the pre-attachment marsupial. In the monotremes there are structural similarities to the progestational condition as seen in viviparous mammals in the luteal phase of the oestrus cycle. The allantois enlarges in the egg of Tachyglossus after the egg is laid, makes contact with the chorion then becomes highly vascular (See Griffiths, 1978). It has been shown to cover half the inner surface of the shell, the vascularised yolk sac covering the remainder of the shell (Semon, 1894).

According to the authors³ most available evidence supports a proposal (Hill, 1900b) that the non-vascular (bilaminar) yolk sac is mainly for respiratory exchange and is the main route of nutrient absorption. After the egg leaves the uterus of Tachyglossus there is no need for a nutritional route but the entire surface becomes vascularised as it essential for respiratory exchange, probably the reason for the vascularisation. Monotremes were excluded from the character 'placenta' in the development of characters for a cladistic analysis (Marshall, 1979), which increases the apparent separation between the 2 groups. The authors³ say the functional reality is much closer. It has been suggested (Gregory, 1947) there is a possibility that there is not a lot of difference between organogenesis continuing for 10 days in the uterus or in an externally held egg, as the products are so similar. The definition of oviparity is not consistent with the egg accumulating so much nutrient material after the shell has been laid down. The authors³ suggest that the term egg has obscured the fundamental similarities, using the term oviparity in relation to monotremes is an obstacle to gaining a better understanding of the essential and close similarities between the reproduction of monotremes and marsupials.

The authors³ suggest the physiological limits of the vascular yolk sac may have been a factor, based on the similar altricial nature of their young. It has been suggested that the evolution of the eutherian villous allantoic placenta allowed a greatly increased exchange to take place thereby allowing the retention of the fetus during its major phase of growth (Luckett, 1977). A tolerance of the fetal tissue by the maternal immune system is necessary for the greatly increased penetration of the maternal tissues and the intimate vascular beds (see Amoroso & Perry, 1975). It has been suggested that as the marsupials didn't evolve a trophoblast that was able to mask histocompatibility antigens on its surface there could be only a brief attachment period in the gestation of marsupials, intolerance of the mother's immune system for the fetal tissues would cause any attachment that was longer to fail (Moors, 1974). It was suggested that this is the reason for the very short-lived chorioallentoic placenta attachment, that is very intimate in Peramelidae (Tyndale-Biscoe, 1973).

The development of the thesis that a trophoblast layer differentiated that was able to mask the histocompatibility antigens on its surface was a major adaptation enabling gestation to be greatly lengthened in eutherian mammals, and that this was the main dichotomy with marsupials (Lillegraven, 1975; Cox, 1977). It has been argued that in marsupials the lack of an extended gestation is not intrinsically inferior to a long uterine gestation, the trophoblast not being a new development in eutheria (Kirsch, 1977b; Taylor & Padykula, 1978). There have been 2 attempts to test the hypothesis that the trophoblast of marsupials does not have the ability to mask histocompatibility antigens failed to support the hypothesis. It has been acknowledged (Lillegraven, 1979) but he suggests that as a result of the species used, Macropus eugenii being derived from an island population the animals may have been closely related so would not provide a good test of the hypothesis. The authors³ suggest that as the island is large 4000 km², the population must be greater than 100,000 individuals, and in both cases the animals rejected allografts of skin in a time period that was normal for outbred (eutherian) animals, indicating this is not a significant distinction between the 2 animal groups.

Birth size

The largest living marsupials are Macropus giganteus, the eastern grey kangaroo, and Macropus rufus, the red kangaroo. Females of these large kangaroos weigh about 28 kg, but at birth their single young weigh about 830 mg, 0.003 % of the weight of the mother, or about half the size of a newborn mouse. A newborn mouse or human weigh about 5 % of the mothers weight. Among the marsupials, the birth weight is about 200-400 mg, some of the smaller dasyurids weigh as little as 10 mg, though the smallest newborn marsupial, the honey possum, is about 4 mg. As with the placental mammals, where the young are much more advanced at birth, the young marsupials control the onset of their own birth. It has been suggested that the small size is the result of the short gestation of marsupials. Some marsupials do have a short gestation period, in some cases less than 2 weeks, but others have gestation periods that are longer than in placentals of similar size. The main difference between the reproductive strategies of marsupials and placentals is the advanced stage of development at birth in placentals, most of the development having taken place in utero, which can occur because of the well-developed placenta. As a result, the young of many placentals are 'ready to roll' a very short time after birth, especially in the case of prey species, where they need to be ready to escape predators as soon as possible, or keep up with the herd. In marsupials, most development takes place after birth. The females of both marsupials and placentals make investments in their young, the placental before birth, the marsupial after birth. In marsupials, the gestation is often short, but the lactation is long and complex, requiring large changes in the quantity and composition of the milk before the young can reach the stage of development attained by placental newborns.

This difference has been exploited by kangaroos, that inhabit some of the driest parts of the driest vegetated continent, with the most erratic climate, the land of 'drought and flooding rain'. When times are bad enough for the milk supply to dry up, they lose any young in the pouch, or out of the pouch but still depending on milk, or the pouch young leaves the pouch, the young that are still at an early stage of development in the uterus that has stopped growing, embryo diapause, resumes development. Because of their method of reproduction, the females of kangaroo species can have one young out of the pouch but suckling, another still-developing young attached to the other teat and the a third still in the uterus that stops development, embryonic diapause, until the more advanced of its larger siblings is weaned, when it resumes development. Each teat produces milk with the composition and volume suitable for the stage of development of the young that feeds from it. When the 'joey' is weaned, the teat it has fed from since its birth 'resets', producing milk with the appropriate composition, and of the appropriate quantity, for the developing young in the uterus to attach to after it is born. This allows the kangaroo population to quickly rebound after it has been reduced by severe conditions, such as a prolonged drought, during which reproduction ceases, but with an embryo that resumes development when required.

Sexual Differentiation - placentals vs marsupials

The external genitalia of marsupials and placentals are superficially similar, but there are differences in development. Sexual differentiation occurs during gestation in the foetus of placentals, at which point the external appearance of both sexes is the same, hence the term 'the indifferent stage'. In male foetuses, the production of testosterone by the developing testes causes the genital tubercle to develop into a penis, and behind it, a scrotum. The same structures develop into a clitoris and the outer lips of the vulva respectively in female foetuses, where testosterone is normally absent. The nipples and mammary glands are formed in both sexes and retained throughout life. The processes that occur after the indifferent stage is controlled by the 'sex determining region', the SRY gene, on the Y chromosome. When this gene is present, as in normal males, it causes the testes to develop from the gonads, the testosterone produced by the testes taking on the orchestration of the changes that occur to produce the rest of the male reproductive structures. The foetus develops as a female if the the SRY gene is absent. In both marsupials and placentals, it is normal for an individual to have 2 X chromosomes in females, and 1 Y chromosome and 1 X chromosome in males. If there is an X chromosome but no Y chromosome, XO, the individual is female. If there are 2 X and a Y chromosome, XXY, the individual is male.

Development of sexual characteristics is different in marsupials, development of the scrotum beginning as 2 bulges in front of the genital tubercle. In female marsupials there is no structure equivalent to the outer lips of the vulva present in female placentals. The pouch and mammary glands don't develop in male marsupials. Male marsupials have a Y chromosome with an SRC gene, which directs the development of the gonads into testes, from which the testosterone secreted leads to the development of internal male genitalia, and the development of the genital tubercle into the male form. The differentiation of mammary pouch and scrotum is not controlled by the developing testes. In genetic males, scrotal bulges develop many days prior to the stage at which the gonads can be distinguished as an ovary or testis, and in genetic females, the mammary glands and pouch also develop many days before the ovary and testis can be distinguished. The later production of sex hormones does not affect these reproductive organs.

It is currently believed that in marsupials the external reproductive organs, such as scrotum, mammary glands or pouch, are probably controlled directly by the sex chromosome constitution of the tissues involved, especially the X chromosome (Cooper, 1993). In normal male marsupials, the X chromosome is involved in the development of scrotal bulges, as well as being involved in the production of the mammary glands and pouch in normal females (Renfrew et al., 1996a). Tammars that are genetically abnormal, XO individuals, have internal female organs but no external female organs, mammary glands or pouch, though they posses a well-developed scrotum, that is empty. XXY tammars have internal male reproductive organs and a well-developed penis, as a result of having a Y chromosome, but they have mammary glands and a small pouch in place of the scrotum, as they have 2 X chromosomes (Sharman et al., 1990).

Physiological Differences - marsupials vs other mammals

As with other mammals and birds, marsupials maintain a constant body temperature, the basal body temperature (BBT), though at a different level than mammals and birds. The BBTs of the different animal groups ranges from 30^o C in monotremes, 35.5^o C in marsupials, 38.0^o C in placentals, 39.5^oC in non passerine birds and 40.5^o C in passerine birds. Reptiles have a BBT of 30^o C that they maintain by moving between sunlight and shade. The reason for the different BBTs of the various groups is not known, though it appears to be genetically determined. The BBT of marsupials has as great an effect on their lives as does their reproductive mode. The body temperature of an animal determines the speed of chemical reactions within the animal, the reaction speed approximately doubling for every 10^o C rise in temperature. According to this, the marsupials standard metabolic rate (SMR) in kj/kg^0.75/day would be about 25 % lower than that of placentals, which are about 25 % less than that of passerines. The SMR of marsupials is 204, that of placentals is 289.

Brain anatomy - placentals vs marsupials

The relationship between marsupial groups, such as the Diprodontia and the Polyprotodontia, that appear similar can be distinguished by studying the links between the 2 halves of the forebrain. In placentals there are 3 commissures in their brains, nerve tracts linking the 2 halves of the brain. The 2 linking the hemispheres of the forebrain are the larger anterior commissure, also linking the 2 olfactory lobes in the forebrain, and the smaller hippocampal commissure (Johnson, 1977). The corpus callosum links the 2 halves of the cerebral cortex. The corpus callosum is lacking in marsupials and monotremes. Among the marsupials, the Diprotodontia have a tract of nerve fibres, the faciculus aberrans, that extends the links of the anterior commissure to link the halves of the cerebral cortex. The Caenolestidae lack this tract, which supports the suggestion that their dentition, similar to that of the Diprotodonts, was probably a case of their dentition arising independently. At the time of writing the brain of Dromiciops had not been studied.

Sperm morphology and anatomy

Relationships among the marsupials can be indicated by the fine structure and morphology of their spermatozoa. The Didelphidae and the Caenolestidae, the 2 main families of American Marsupials, the sperm occur as conjoined pairs, the heads of the 2 spermatozoa being pressed closely together, remaining in this state until reaching the vicinity of the egg in the female's oviduct. Such conjugation is unknown in any Australian marsupial, and is not known of in any placental. Unlike other American marsupials, Dromiciops also has sperm that is not conjugated, as is the case with Australian marsupials, adding further support to the notion that there were close links between the Australian and American marsupials.

Sex chromosomes

In mammals, the sex chromosomes are a single pair of chromosomes carrying genes determining the sexual differentiation. The X chromosome, usually of normal size, also contains many genes not directly connected with sexual differentiation. The Y chromosome is usually very small and contains few genes not connected with sex determination. In most marsupials the X chromosomes are small, comprising less than 3 % of the total DNA, and even smaller Y chromosomes that don't pair with the corresponding X chromosome. In placentals, the X chromosome has more than 5 % of the total DNA, and the Y chromosome has some genes that are also present on the X chromosome, and pairs with the X chromosome during meiosis. It has been suggested that the marsupial pattern is the original, primitive condition, additional sections of an autosome becoming attached to the X and Y chromosomes at some early stage in the evolution of placentals (Jennifer Graves, 1996). 4 marsupial species have a large X chromosome, so presumably a similar process occurred in them independently. The female potoroo (Potorous tredactylus), has 10 autosomes and 2 large X chromosomes. Males of this species have 10 autosomes, 1 large X chromosome, as well as 1 large Y and 1 small Y chromosome. The large Y chromosome is the remaining half of the autosome that fused with the original X chromosome, pairing at meiosis with 1 arm of the Large X chromosome, the small Y chromosome pairing with the other arm of the X chromosome.

It is believed a similar process occurred in the swamp wallaby, with 8 autosomes, 1 large X chromosome, 1 large Y and 1 small Y chromosome. The sequence of events involved has been determined by the use of chromosome painting, the addition of antibodies to the chromosomes of tammar wallabies. The long arm of the X chromosome and the whole of the large Y chromosome are homologous with chromosomes 2 and 7 of the tammar, while only the short arm of the the X is homologous with the X chromosome of the tammar (Toder et al., 1997). Closely related species have the normal XX/XY arrangement of sex chromosomes, indicating that the unusual arrangements don't appear to have a noticeable effect on the aberrant species.

X-chromosome inactivation

In mammals the females have 2 X chromosomes, 1 from each parent, each of which has the same complement of genes, so the females get 2 of every gene, whereas the males have a single copy of each X chromosome gene, that invariably came from their mother. One of the X chromosomes is inactivated in the females of both marsupials and placentals, and replicates later than the active X chromosome. The X chromosome that is inactivated, in any cell, may be from either parent in placentals. Hence the term random X inactivation. In marsupials it is nearly always the X chromosome from the father that is inactivated - paternal X inactivation. This aspect of marsupial genetics has been studied most in the kangaroos, but some evidence exists for this process occurring in other families. Most tissues in bandicoot bodies have only 1 X chromosome, during development, in females the second X and the Y chromosome in males is lost during development. The full sex chromosome complement is retained only in the tissues of the gonads (Hayman, 1990) - sex chromosome mosaicism.

The chromosome number and arrangement supports the relationships among marsupials that have been determined based on other criteria, but the many changes in the number and order of the chromosomes mean it is hard to interpret, being of more use to support conclusions that are made based on other criteria, such as protein and DNA sequences.

Sources & Further reading

Chris Johnson, Australia's Mammal Extinctions, a 50,000 year history, Cambridge University Press, 2006
Tyndale-Biscoe, Hugh, 2005, Life of Marsupials, CSIRO Publishing.
Tyndale-Biscoe, Hugh & Renfree, Marilyn, 1987, Reproductive Physiology of Marsupials, Cambridge University Press.

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