Surrey Amphibian and Reptile Group (SARG) 

Water Frog Genetics

  

Water Frog Interbreeding

Introduction - Some basic biology

Gametes are cells which are connected with sexual reproduction, which are produced from adult cells through a process called meiosis. Male frog gametes are sperm, and female frog gametes are eggs. Each gamete contains a single sex chromosome, chosen at random from one of the pair of adult sex chromosomes. Male frogs have an X-chromosome and a Y-chromosome, whilst female frogs have two X-chromosomes.

Offspring share one chromosome from their mother, and one from their father. The choice of chromosome is random. The combinations of chromosomes provide the key to understanding the interbreeding results of water frog species.

Water Frog Systems

There are at least four types of Edible frog, which are indistinguishable from eachother, except by analysing their offspring. Depending upon the combination of other species present in their habitat, certain types of Edible frog prove non-viable in the long term.

In Britain and Western Europe, the only common combination of Water Frog species is called the L-E system (L= lessonae[Pool frog], E = esculenta [Edible frog]). This system comprises Pool frogs and Edible frogs of both sexes, and a small number of female Marsh frogs.

The L-E System

Key to table 2.

Cells with a black background are frequent events.
Cells with a blue background are infrequent events, or do not occur.

The first letter, in capitals represents the species (P=pool frog, M=marsh frog & E=edible frog)
The second letter, in lower case, represents either the X chromosome, or the Y chromosome.
Chromosomes from the father are in blue, chromosomes from the mother are in yellow.
Chromosomes that are cloned are shown in red.

Example: Px is an X-chromosome from a male Pool frog.
P = Pool frog, x = X-chromosome, blue = from the father.

♂ = male
♀ = female

Table 2
All possible diploid
zygote combinations
(L-E System)

Male Pool frog
PyPx
 

Male Edible frog
PyMx
 

Male Marsh frog
MyMx
[not present in L-E system]1
Gametes: Py Px Mx2 Mx My Mx

Female Pool frog
PxPx
Px
♂ Pool frog
PyPx

♀ Pool frog
PxPx
3
♀ Edible frog
MxPx

♀ Edible frog
MxPx
4
♂ Edible frog
MyPx

♀ Edible frog
MxPx
Px
♂ Pool frog
PyPx

♀ Pool frog
PxPx

♀ Edible frog
MxPx

♀ Edible frog
MxPx

♂ Edible frog
MyPx

♀ Edible frog
MxPx

Female Edible frog
PxMx
Mx5
♂ Edible frog
PyMx

♀ Edible frog
PxMx
6
♀ Marsh frog
MxMx

♀ Marsh frog
MxMx
7
♂ Marsh frog
MyMx

♀ Marsh frog
MxMx
Mx
♂ Edible frog
PyMx

♀ Edible frog
PxMx

♀ Marsh frog
MxMx

♀ Marsh frog
MxMx

♂Marsh frog
MyMx

♀ Marsh frog
MxMx
8
Female Marsh frog
MxMx
Mx 9
♂ Edible frog
PyMx

♀ Edible frog
PxMx
10
♀ Marsh frog
MxMx

♀ Marsh frog
MxMx
11
♂Marsh frog
MyMx

♀ Marsh frog
MxMx
Mx
♂ Edible frog
PyMx

♀ Edible frog
PxMx

♀ Marsh frog
MxMx

♀ Marsh frog
MxMx

♂ Marsh frog
MyMx

♀ Marsh frog
MxMx

Notes:

1

Pool frogs and Marsh frogs have different preferences for habitat. Pool frogs prefer small, well vegetated pools, whilst the Marsh frog has a preference for open water bodies with little aquatic vegetation. Although the two species can co-exist within the same environment, it is more usual that one species dominates. In the L-E system, Pool frogs dominate, and co-exist with Edible frogs (which have habitat preferences between Pool and Marsh frogs). Hence, Marsh frogs are either not present, or present in only small numbers within a classic L-E system.

2

Although the male Edible frog has X-chromasomes within its Marsh frog genome component, and a Y-chromasome from its Pool frog genome, during the production of gametes (sperm) the Pool frog genome is supressed, and replaced by a clone of the Marsh frog genome. Hence the male Edible frog is incapable of passing on Y-chromasomes to its offspring, resulting in all offspring being female.

3

In practice, female Pool frogs are not observed to mate with male Edible frogs. Why this is so is not well understood, but is believed to be a mate-selection issue. It has been observed that male Pool frogs are very fast to couple with Pool frog females, and may beat male Edible frogs to amplexus. In captivity, given no other option, female Pool frogs will mate with a male Edible frog of suitable size, but this is a rare occurence in the wild. Hence, the main mechanism for Pool-Edible interbreeding is via male Pool frogs and female Edible frogs.

4

In an L-E system, this combination does not occur due to the absence of male Marsh frogs. However, should male Marsh frogs be present, their large size in comparisson to the female Pool frog, means that mate selection rarely, if ever, occurs between this combination.

5

Although the female Edible frog has X-chromasomes within its Marsh frog genome component, and a X-chromasome from its Pool frog genome, during the production of gametes (eggs) the Pool frog genome is supressed, and replaced by a clone of the Marsh frog genome. As female Edible frogs are produced mainly from the combination of male Pool frogs and female Edible frogs (in the L-E system), then the Marsh frog genome is already a cloned genome in the adult. The production of eggs by female Marsh frogs crates another clone, of this already pre-cloned genome.

6

It can be deduced from the table, that all offspring of Edible frog male and female mating (in an L-E system) are female. In addition, all genomes of this female progeny are cloned. This leads to an accumulation of deleterious mutations of the cloned Marsh frog genomes, and very few survive to metamophosis. However, a small number of animals do survive, which leads to the few female Marsh frogs occasionally found within L-E systems.

7

Although male Marsh frogs are not usually present in an L-E system, even should any occur, this is not a combination that occurs in the wild. Female Edible frogs are strongly drawn to the calls of Pool frogs, and not by the call of male Marsh frogs. This makes prefect evolutionary sense, as should the female Edible frog mate with a male Pool frog, then more Edible frogs are the result. If however they were to mate with Marsh frog males, the offspring would be Marsh frogs, possibly leading to the Edible frog breeding itself out of existence.

8

Although Marsh frogs are not normally a part of the L-E system, we have shown that a few surviving female Marsh frogs are produced from the coupling between Edible frogs. This is the usual source of any female Marsh frogs in an L-E system.

9

This is a viable combination, and is the most likely combination that produces first generation Edible frogs. Male frogs tendency to be attracted to large female frogs also leads to this combination being a likely occurrence. The female Marsh frog is less likely to be attracted to the calls by male Pool frogs than from males of their own species, but given the normal absence of Marsh frog males in an L-E system, this is possible. It occurs less frequently than other combinations due to the low numbers of female Marsh frogs within the L-E system, which is in turn due to the poor survival rate of female Marsh frogs that are the progeny of mating between Edible frogs.

10

Note - This section is a description of logical conclusions from the interbreeding model, and has not been verified from observation. This combination is likely to occur, as the male Edible frog would be attracted to the larger Marsh frog female. Additionally, in evolutionary terms, if a female Marsh frog responds to the call of a male Edible frog, their progeny would be female Marsh frogs, so enhancing their species. As offspring are formed from entirely cloned genomes, their chances of survival to metamorphosis is slim at best. This would be an infrequent occurrence due to the very small numbers of female Marsh frogs within the L-E system.

11

This is an infrequent event at best, due to the normal absence of male Marsh frogs within the L-E system.