User:Manudouz/sandbox/L95 model

L95, a model of DNA evolution proposed by Jean Lobry in 1995, is a general model under no-strand-bias conditions, i.e., when mutation and selection do have the same effect on each of the two complementary DNA strands. It incorporates Watson and Crick base pairing rules: the exchange rate from a nucleotide $i$ to another $j$ is equal to the rate from ${\bar {i}}$ (i.e., the complement of $i$) towards ${\bar {j}}$ (i.e., the complement of $j$). The model therefore reduces to six the number of exchange rates between nucleotides. Note that this model is not time-reversible.

L95 parameters consist of an equilibrium base frequency vector, $\Pi =(\pi _{T},\pi _{C},\pi _{A},\pi _{G})$, giving the frequency at which each base occurs at each site, and the rate matrix where exchangeabilities between pairing bases are equal. For example $\gamma$ is the A $\rightarrow$ G exchange rate, and it is equal to the exchange rate from T (the complementary base of A) to C (the complementary base of G): $\gamma =r(A\rightarrow G)=r({\bar {A}}\rightarrow {\bar {G}})=r(T\rightarrow C)$. The rationale behind this is the fact that a mutation on one strand introduces a mismatch, prompting the occurrence / favoring a second mutation on the complementary strand to compensate for the first one / possibly prompting the DNA mismatch repair on the complementary strand:

${\begin{matrix}&{\mathsf {A}}&&{\mathsf {G}}&&{\mathsf {G}}\\{\mathsf {(1)\ Initial\ pair:}}&\parallel &{\mathsf {\quad (2)\ A\ \rightarrow \ G\ \ change\ on\ the\ first\ strand:\ }}&\nparallel &{\mathsf {\quad (3)\ T\ \rightarrow \ C\ \ compensating\ change\ on\ the\ complementary\ strand:\ }}&\parallel \\&{\mathsf {T}}&&{\mathsf {T}}&&{\mathsf {C}}\\\end{matrix}}$

Over long evolutionary times, the A $\rightarrow$ G exchange rate would therefore equate the T $\rightarrow$ C exchange rate.

Sueoka

By columns

$\qquad \ {\mathsf {From}}\qquad \quad {\begin{matrix}\ \ \ {\mathsf {T}}&\qquad \qquad \qquad \qquad \ \ {\mathsf {C}}&\qquad \qquad \qquad \qquad \ \ \ {\mathsf {A}}&\qquad \qquad \qquad \qquad \ \ {\mathsf {G}}\\\end{matrix}}$

$Q={\begin{pmatrix}{-(\gamma \pi _{C}+\alpha \pi _{A}+\epsilon \pi _{G})}&{\beta \pi _{T}}&{\alpha \pi _{T}}&{\delta \pi _{T}}\\{\gamma \pi _{C}}&{-(\beta \pi _{T}+\delta \pi _{A}+\eta \pi _{G})}&{\epsilon \pi _{C}}&{\eta \pi _{C}}\\{\alpha \pi _{A}}&{\delta \pi _{A}}&{-(\alpha \pi _{T}+\epsilon \pi _{C}+\gamma \pi _{G})}&{\beta \pi _{A}}\\{\epsilon \pi _{G}}&{\eta \pi _{G}}&{\gamma \pi _{G}}&{-(\delta \pi _{T}+\eta \pi _{C}+\beta \pi _{A})}\\\end{pmatrix}}\ \ {\mathsf {to}}\ \ {\begin{matrix}{\mathsf {T}}\\{\mathsf {C}}\\{\mathsf {A}}\\{\mathsf {G}}\\\end{matrix}}$

By rows | T C A G

$\qquad \qquad \qquad \qquad \ {\mathsf {to}}\qquad \quad \quad {\begin{matrix}\ \ \ \ {\mathsf {T}}&\qquad \qquad \qquad \qquad \ \ \ {\mathsf {C}}&\qquad \qquad \qquad \qquad \ \ \ {\mathsf {A}}&\qquad \qquad \qquad \qquad \ \ \ {\mathsf {G}}\\\end{matrix}}$

${\mathsf {From}}\ \ {\begin{matrix}{\mathsf {T}}\\{\mathsf {C}}\\{\mathsf {A}}\\{\mathsf {G}}\\\end{matrix}}\qquad Q={\begin{pmatrix}{-(\gamma \pi _{C}+\alpha \pi _{A}+\epsilon \pi _{G})}&{\gamma \pi _{C}}&{\alpha \pi _{A}}&{\epsilon \pi _{G}}\\{\beta \pi _{T}}&{-(\beta \pi _{T}+\delta \pi _{A}+\eta \pi _{G})}&{\delta \pi _{A}}&{\eta \pi _{G}}\\{\alpha \pi _{T}}&{\epsilon \pi _{C}}&{-(\alpha \pi _{T}+\epsilon \pi _{C}+\gamma \pi _{G})}&{\gamma \pi _{G}}\\{\delta \pi _{T}}&{\eta \pi _{C}}&{\beta \pi _{A}}&{-(\delta \pi _{T}+\eta \pi _{C}+\beta \pi _{A})}\\\end{pmatrix}}$

By rows | A G C T

$\qquad \qquad \qquad \qquad \ {\mathsf {to}}\qquad \quad \quad {\begin{matrix}\ \ \ \ {\mathsf {A}}&\qquad \qquad \qquad \qquad \ \ \ {\mathsf {G}}&\qquad \qquad \qquad \qquad \ \ \ {\mathsf {C}}&\qquad \qquad \qquad \qquad \ \ \ {\mathsf {T}}\\\end{matrix}}$

${\mathsf {From}}\ \ {\begin{matrix}{\mathsf {A}}\\{\mathsf {G}}\\{\mathsf {C}}\\{\mathsf {T}}\\\end{matrix}}\qquad Q={\begin{pmatrix}{-(\gamma \pi _{G}+\epsilon \pi _{C}+\alpha \pi _{T})}&{\gamma \pi _{G}}&{\epsilon \pi _{C}}&{\alpha \pi _{T}}\\{\beta \pi _{A}}&{-(\beta \pi _{A}+\eta \pi _{C}+\delta \pi _{T})}&{\eta \pi _{C}}&{\delta \pi _{T}}\\{\delta \pi _{A}}&{\eta \pi _{G}}&{-(\delta \pi _{A}+\eta \pi _{G}+\beta \pi _{T})}&{\beta \pi _{T}}\\{\alpha \pi _{A}}&{\epsilon \pi _{G}}&{\gamma \pi _{C}}&{-(\alpha \pi _{A}+\epsilon \pi _{G}+\gamma \pi _{C})}\\\end{pmatrix}}$

See also

• Substitution model
• Detailed balance property